Docker build from dockerfile windows

Docker can build images automatically by reading the instructions from a
Dockerfile. A Dockerfile is a text document that contains all the commands a
user could call on the command line to assemble an image. Using docker build
users can create an automated build that executes several command-line
instructions in succession.

This page describes the commands you can use in a Dockerfile. When you are
done reading this page, refer to the Dockerfile Best
Practices for a tip-oriented guide.

Usage

The docker build command builds an image from
a Dockerfile and a context. The build’s context is the files at a specified
location PATH or URL. The PATH is a directory on your local filesystem.
The URL is a the location of a Git repository.

A context is processed recursively. So, a PATH includes any subdirectories and
the URL includes the repository and its submodules. A simple build command
that uses the current directory as context:

$ docker build .
Sending build context to Docker daemon  6.51 MB
...

The build is run by the Docker daemon, not by the CLI. The first thing a build
process does is send the entire context (recursively) to the daemon. In most
cases, it’s best to start with an empty directory as context and keep your
Dockerfile in that directory. Add only the files needed for building the
Dockerfile.

Warning: Do not use your root directory, /, as the PATH as it causes
the build to transfer the entire contents of your hard drive to the Docker
daemon.

To use a file in the build context, the Dockerfile refers to the file specified
in an instruction, for example, a COPY instruction. To increase the build’s
performance, exclude files and directories by adding a .dockerignore file to
the context directory. For information about how to create a .dockerignore
file see the documentation on this page.

Traditionally, the Dockerfile is called Dockerfile and located in the root
of the context. You use the -f flag with docker build to point to a Dockerfile
anywhere in your file system.

$ docker build -f /path/to/a/Dockerfile .

You can specify a repository and tag at which to save the new image if
the build succeeds:

$ docker build -t shykes/myapp .

To tag the image into multiple repositories after the build,
add multiple -t parameters when you run the build command:

$ docker build -t shykes/myapp:1.0.2 -t shykes/myapp:latest .

The Docker daemon runs the instructions in the Dockerfile one-by-one,
committing the result of each instruction
to a new image if necessary, before finally outputting the ID of your
new image. The Docker daemon will automatically clean up the context you
sent.

Note that each instruction is run independently, and causes a new image
to be created — so RUN cd /tmp will not have any effect on the next
instructions.

Whenever possible, Docker will re-use the intermediate images (cache),
to accelerate the docker build process significantly. This is indicated by
the Using cache message in the console output.
(For more information, see the Build cache section) in the
Dockerfile best practices guide:

$ docker build -t svendowideit/ambassador .
Sending build context to Docker daemon 15.36 kB
Step 1 : FROM alpine:3.2
 ---> 31f630c65071
Step 2 : MAINTAINER SvenDowideit@home.org.au
 ---> Using cache
 ---> 2a1c91448f5f
Step 3 : RUN apk update &&      apk add socat &&        rm -r /var/cache/
 ---> Using cache
 ---> 21ed6e7fbb73
Step 4 : CMD env | grep _TCP= | (sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat -t 100000000 TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/' && echo wait) | sh
 ---> Using cache
 ---> 7ea8aef582cc
Successfully built 7ea8aef582cc

When you’re done with your build, you’re ready to look into Pushing a
repository to its registry.

Format

Here is the format of the Dockerfile:

# Comment
INSTRUCTION arguments

The instruction is not case-sensitive. However, convention is for them to
be UPPERCASE to distinguish them from arguments more easily.

Docker runs instructions in a Dockerfile in order. The first
instruction must be `FROM` in order to specify the Base
Image from which you are building.

Docker treats lines that begin with # as a comment, unless the line is
a valid parser directive. A # marker anywhere
else in a line is treated as an argument. This allows statements like:

# Comment
RUN echo 'we are running some # of cool things'

Line continuation characters are not supported in comments.

Parser directives

Parser directives are optional, and affect the way in which subsequent lines
in a Dockerfile are handled. Parser directives do not add layers to the build,
and will not be shown as a build step. Parser directives are written as a
special type of comment in the form # directive=value. A single directive
may only be used once.

Once a comment, empty line or builder instruction has been processed, Docker
no longer looks for parser directives. Instead it treats anything formatted
as a parser directive as a comment and does not attempt to validate if it might
be a parser directive. Therefore, all parser directives must be at the very
top of a Dockerfile.

Parser directives are not case-sensitive. However, convention is for them to
be lowercase. Convention is also to include a blank line following any
parser directives. Line continuation characters are not supported in parser
directives.

Due to these rules, the following examples are all invalid:

Invalid due to line continuation:

# direc \
tive=value

Invalid due to appearing twice:

# directive=value1
# directive=value2

FROM ImageName

Treated as a comment due to appearing after a builder instruction:

FROM ImageName
# directive=value

Treated as a comment due to appearing after a comment which is not a parser
directive:

# About my dockerfile
FROM ImageName
# directive=value

The unknown directive is treated as a comment due to not being recognized. In
addition, the known directive is treated as a comment due to appearing after
a comment which is not a parser directive.

# unknowndirective=value
# knowndirective=value

Non line-breaking whitespace is permitted in a parser directive. Hence, the
following lines are all treated identically:

#directive=value
# directive =value
#	directive= value
# directive = value
#	  dIrEcTiVe=value

The following parser directive is supported:

• escape

escape

Or

The escape directive sets the character used to escape characters in a
Dockerfile. If not specified, the default escape character is \.

The escape character is used both to escape characters in a line, and to
escape a newline. This allows a Dockerfile instruction to
span multiple lines. Note that regardless of whether the escape parser
directive is included in a Dockerfile, escaping is not performed in
a RUN command, except at the end of a line.

Setting the escape character to ` is especially useful on
Windows, where \ is the directory path separator. ` is consistent
with Windows PowerShell.

Consider the following example which would fail in a non-obvious way on
Windows. The second \ at the end of the second line would be interpreted as an
escape for the newline, instead of a target of the escape from the first \.
Similarly, the \ at the end of the third line would, assuming it was actually
handled as an instruction, cause it be treated as a line continuation. The result
of this dockerfile is that second and third lines are considered a single
instruction:

FROM windowsservercore
COPY testfile.txt c:\\
RUN dir c:\

Results in:

PS C:\John> docker build -t cmd .
Sending build context to Docker daemon 3.072 kB
Step 1 : FROM windowsservercore
 ---> dbfee88ee9fd
Step 2 : COPY testfile.txt c:RUN dir c:
GetFileAttributesEx c:RUN: The system cannot find the file specified.
PS C:\John>

One solution to the above would be to use / as the target of both the COPY
instruction, and dir. However, this syntax is, at best, confusing as it is not
natural for paths on Windows, and at worst, error prone as not all commands on
Windows support / as the path separator.

By adding the escape parser directive, the following Dockerfile succeeds as
expected with the use of natural platform semantics for file paths on Windows:

# escape=`

FROM windowsservercore
COPY testfile.txt c:\
RUN dir c:\

Results in:

PS C:\John> docker build -t succeeds --no-cache=true .
Sending build context to Docker daemon 3.072 kB
Step 1 : FROM windowsservercore
 ---> dbfee88ee9fd
Step 2 : COPY testfile.txt c:\
 ---> 99ceb62e90df
Removing intermediate container 62afbe726221
Step 3 : RUN dir c:\
 ---> Running in a5ff53ad6323
 Volume in drive C has no label.
 Volume Serial Number is 1440-27FA

 Directory of c:\

03/25/2016  05:28 AM    <DIR>          inetpub
03/25/2016  04:22 AM    <DIR>          PerfLogs
04/22/2016  10:59 PM    <DIR>          Program Files
03/25/2016  04:22 AM    <DIR>          Program Files (x86)
04/18/2016  09:26 AM                 4 testfile.txt
04/22/2016  10:59 PM    <DIR>          Users
04/22/2016  10:59 PM    <DIR>          Windows
               1 File(s)              4 bytes
               6 Dir(s)  21,252,689,920 bytes free
 ---> 2569aa19abef
Removing intermediate container a5ff53ad6323
Successfully built 2569aa19abef
PS C:\John>

Environment replacement

Environment variables (declared with the ENV statement) can also be
used in certain instructions as variables to be interpreted by the
Dockerfile. Escapes are also handled for including variable-like syntax
into a statement literally.

Environment variables are notated in the Dockerfile either with
$variable_name or ${variable_name}. They are treated equivalently and the
brace syntax is typically used to address issues with variable names with no
whitespace, like ${foo}_bar.

The ${variable_name} syntax also supports a few of the standard bash
modifiers as specified below:

• ${variable:-word} indicates that if variable is set then the result
  will be that value. If variable is not set then word will be the result.
• ${variable:+word} indicates that if variable is set then word will be
  the result, otherwise the result is the empty string.

In all cases, word can be any string, including additional environment
variables.

Escaping is possible by adding a \ before the variable: \$foo or \${foo},
for example, will translate to $foo and ${foo} literals respectively.

Example (parsed representation is displayed after the #):

FROM busybox
ENV foo /bar
WORKDIR ${foo}   # WORKDIR /bar
ADD . $foo       # ADD . /bar
COPY \$foo /quux # COPY $foo /quux

Environment variables are supported by the following list of instructions in
the Dockerfile:

• ADD
• COPY
• ENV
• EXPOSE
• LABEL
• USER
• WORKDIR
• VOLUME
• STOPSIGNAL

as well as:

• ONBUILD (when combined with one of the supported instructions above)

Note:
prior to 1.4, ONBUILD instructions did NOT support environment
variable, even when combined with any of the instructions listed above.

Environment variable substitution will use the same value for each variable
throughout the entire command. In other words, in this example:

ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc

will result in def having a value of hello, not bye. However,
ghi will have a value of bye because it is not part of the same command
that set abc to bye.

.dockerignore file

Before the docker CLI sends the context to the docker daemon, it looks
for a file named .dockerignore in the root directory of the context.
If this file exists, the CLI modifies the context to exclude files and
directories that match patterns in it. This helps to avoid
unnecessarily sending large or sensitive files and directories to the
daemon and potentially adding them to images using ADD or COPY.

The CLI interprets the .dockerignore file as a newline-separated
list of patterns similar to the file globs of Unix shells. For the
purposes of matching, the root of the context is considered to be both
the working and the root directory. For example, the patterns
/foo/bar and foo/bar both exclude a file or directory named bar
in the foo subdirectory of PATH or in the root of the git
repository located at URL. Neither excludes anything else.

If a line in .dockerignore file starts with # in column 1, then this line is
considered as a comment and is ignored before interpreted by the CLI.

Here is an example .dockerignore file:

# comment
    */temp*
    */*/temp*
    temp?

This file causes the following build behavior:

Matching is done using Go’s
filepath.Match rules. A
preprocessing step removes leading and trailing whitespace and
eliminates . and .. elements using Go’s
filepath.Clean. Lines
that are blank after preprocessing are ignored.

Beyond Go’s filepath.Match rules, Docker also supports a special
wildcard string ** that matches any number of directories (including
zero). For example, **/*.go will exclude all files that end with .go
that are found in all directories, including the root of the build context.

Lines starting with ! (exclamation mark) can be used to make exceptions
to exclusions. The following is an example .dockerignore file that
uses this mechanism:

All markdown files except README.md are excluded from the context.

The placement of ! exception rules influences the behavior: the last
line of the .dockerignore that matches a particular file determines
whether it is included or excluded. Consider the following example:

    *.md
    !README*.md
    README-secret.md

No markdown files are included in the context except README files other than
README-secret.md.

Now consider this example:

    *.md
    README-secret.md
    !README*.md

All of the README files are included. The middle line has no effect because
!README*.md matches README-secret.md and comes last.

You can even use the .dockerignore file to exclude the Dockerfile
and .dockerignore files. These files are still sent to the daemon
because it needs them to do its job. But the ADD and COPY commands
do not copy them to the image.

Finally, you may want to specify which files to include in the
context, rather than which to exclude. To achieve this, specify * as
the first pattern, followed by one or more ! exception patterns.

Note: For historical reasons, the pattern . is ignored.

FROM

Or

Or

The FROM instruction sets the Base Image
for subsequent instructions. As such, a valid Dockerfile must have FROM as
its first instruction. The image can be any valid image – it is especially easy
to start by pulling an image from the Public Repositories.

• FROM must be the first non-comment instruction in the Dockerfile.

• FROM can appear multiple times within a single Dockerfile in order to create
  multiple images. Simply make a note of the last image ID output by the commit
  before each new FROM command.

• The tag or digest values are optional. If you omit either of them, the builder
  assumes a latest by default. The builder returns an error if it cannot match
  the tag value.

MAINTAINER

The MAINTAINER instruction allows you to set the Author field of the
generated images.

RUN

RUN has 2 forms:

• RUN <command> (shell form, the command is run in a shell, which by
  default is /bin/sh -c on Linux or cmd /S /C on Windows)
• RUN ["executable", "param1", "param2"] (exec form)

The RUN instruction will execute any commands in a new layer on top of the
current image and commit the results. The resulting committed image will be
used for the next step in the Dockerfile.

Layering RUN instructions and generating commits conforms to the core
concepts of Docker where commits are cheap and containers can be created from
any point in an image’s history, much like source control.

The exec form makes it possible to avoid shell string munging, and to RUN
commands using a base image that does not contain the specified shell executable.

The default shell for the shell form can be changed using the SHELL
command.

In the shell form you can use a \ (backslash) to continue a single
RUN instruction onto the next line. For example, consider these two lines:

RUN /bin/bash -c 'source $HOME/.bashrc ;\
echo $HOME'

Together they are equivalent to this single line:

RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'


Note:
To use a different shell, other than ‘/bin/sh’, use the exec form
passing in the desired shell. For example,
RUN ["/bin/bash", "-c", "echo hello"]

Note:
The exec form is parsed as a JSON array, which means that
you must use double-quotes (“) around words not single-quotes (‘).

Note:
Unlike the shell form, the exec form does not invoke a command shell.
This means that normal shell processing does not happen. For example,
RUN [ "echo", "$HOME" ] will not do variable substitution on $HOME.
If you want shell processing then either use the shell form or execute
a shell directly, for example: RUN [ "sh", "-c", "echo $HOME" ].
When using the exec form and executing a shell directly, as in the case for
the shell form, it is the shell that is doing the environment variable
expansion, not docker.

Note:
In the JSON form, it is necessary to escape backslashes. This is
particularly relevant on Windows where the backslash is the path separator.
The following line would otherwise be treated as shell form due to not
being valid JSON, and fail in an unexpected way:
RUN ["c:\windows\system32\tasklist.exe"]
The correct syntax for this example is:
RUN ["c:\\windows\\system32\\tasklist.exe"]

The cache for RUN instructions isn’t invalidated automatically during
the next build. The cache for an instruction like
RUN apt-get dist-upgrade -y will be reused during the next build. The
cache for RUN instructions can be invalidated by using the --no-cache
flag, for example docker build --no-cache.

See the Dockerfile Best Practices
guide for more information.

The cache for RUN instructions can be invalidated by ADD instructions. See
below for details.

Known issues (RUN)

• Issue 783 is about file
  permissions problems that can occur when using the AUFS file system. You
  might notice it during an attempt to rm a file, for example.

  For systems that have recent aufs version (i.e., dirperm1 mount option can
  be set), docker will attempt to fix the issue automatically by mounting
  the layers with dirperm1 option. More details on dirperm1 option can be
  found at aufs man page

  If your system doesn’t have support for dirperm1, the issue describes a workaround.

CMD

The CMD instruction has three forms:

• CMD ["executable","param1","param2"] (exec form, this is the preferred form)
• CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
• CMD command param1 param2 (shell form)

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD
then only the last CMD will take effect.

The main purpose of a CMD is to provide defaults for an executing
container. These defaults can include an executable, or they can omit
the executable, in which case you must specify an ENTRYPOINT
instruction as well.

Note:
If CMD is used to provide default arguments for the ENTRYPOINT
instruction, both the CMD and ENTRYPOINT instructions should be specified
with the JSON array format.

Note:
The exec form is parsed as a JSON array, which means that
you must use double-quotes (“) around words not single-quotes (‘).

Note:
Unlike the shell form, the exec form does not invoke a command shell.
This means that normal shell processing does not happen. For example,
CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME.
If you want shell processing then either use the shell form or execute
a shell directly, for example: CMD [ "sh", "-c", "echo $HOME" ].
When using the exec form and executing a shell directly, as in the case for
the shell form, it is the shell that is doing the environment variable
expansion, not docker.

When used in the shell or exec formats, the CMD instruction sets the command
to be executed when running the image.

If you use the shell form of the CMD, then the <command> will execute in
/bin/sh -c:

FROM ubuntu
CMD echo "This is a test." | wc -

If you want to run your <command> without a shell then you must
express the command as a JSON array and give the full path to the executable.
This array form is the preferred format of CMD. Any additional parameters
must be individually expressed as strings in the array:

FROM ubuntu
CMD ["/usr/bin/wc","--help"]

If you would like your container to run the same executable every time, then
you should consider using ENTRYPOINT in combination with CMD. See
ENTRYPOINT.

If the user specifies arguments to docker run then they will override the
default specified in CMD.

Note:
don’t confuse RUN with CMD. RUN actually runs a command and commits
the result; CMD does not execute anything at build time, but specifies
the intended command for the image.

LABEL

LABEL <key>=<value> <key>=<value> <key>=<value> ...

The LABEL instruction adds metadata to an image. A LABEL is a
key-value pair. To include spaces within a LABEL value, use quotes and
backslashes as you would in command-line parsing. A few usage examples:

LABEL "com.example.vendor"="ACME Incorporated"
LABEL com.example.label-with-value="foo"
LABEL version="1.0"
LABEL description="This text illustrates \
that label-values can span multiple lines."

An image can have more than one label. To specify multiple labels,
Docker recommends combining labels into a single LABEL instruction where
possible. Each LABEL instruction produces a new layer which can result in an
inefficient image if you use many labels. This example results in a single image
layer.

LABEL multi.label1="value1" multi.label2="value2" other="value3"

The above can also be written as:

LABEL multi.label1="value1" \
      multi.label2="value2" \
      other="value3"

Labels are additive including LABELs in FROM images. If Docker
encounters a label/key that already exists, the new value overrides any previous
labels with identical keys.

To view an image’s labels, use the docker inspect command.

"Labels": {
    "com.example.vendor": "ACME Incorporated"
    "com.example.label-with-value": "foo",
    "version": "1.0",
    "description": "This text illustrates that label-values can span multiple lines.",
    "multi.label1": "value1",
    "multi.label2": "value2",
    "other": "value3"
},

EXPOSE

EXPOSE <port> [<port>...]

The EXPOSE instruction informs Docker that the container listens on the
specified network ports at runtime. EXPOSE does not make the ports of the
container accessible to the host. To do that, you must use either the -p flag
to publish a range of ports or the -P flag to publish all of the exposed
ports. You can expose one port number and publish it externally under another
number.

To set up port redirection on the host system, see using the -P
flag. The Docker network feature supports
creating networks without the need to expose ports within the network, for
detailed information see the overview of this
feature).

ENV

ENV <key> <value>
ENV <key>=<value> ...

The ENV instruction sets the environment variable <key> to the value
<value>. This value will be in the environment of all “descendant”
Dockerfile commands and can be replaced inline in
many as well.

The ENV instruction has two forms. The first form, ENV <key> <value>,
will set a single variable to a value. The entire string after the first
space will be treated as the <value> — including characters such as
spaces and quotes.

The second form, ENV <key>=<value> ..., allows for multiple variables to
be set at one time. Notice that the second form uses the equals sign (=)
in the syntax, while the first form does not. Like command line parsing,
quotes and backslashes can be used to include spaces within values.

For example:

ENV myName="John Doe" myDog=Rex\ The\ Dog \
    myCat=fluffy

and

ENV myName John Doe
ENV myDog Rex The Dog
ENV myCat fluffy

will yield the same net results in the final container, but the first form
is preferred because it produces a single cache layer.

The environment variables set using ENV will persist when a container is run
from the resulting image. You can view the values using docker inspect, and
change them using docker run --env <key>=<value>.

Note:
Environment persistence can cause unexpected side effects. For example,
setting ENV DEBIAN_FRONTEND noninteractive may confuse apt-get
users on a Debian-based image. To set a value for a single command, use
RUN <key>=<value> <command>.

ADD

ADD has two forms:

• ADD <src>... <dest>
• ADD ["<src>",... "<dest>"] (this form is required for paths containing
  whitespace)

The ADD instruction copies new files, directories or remote file URLs from <src>
and adds them to the filesystem of the container at the path <dest>.

Multiple <src> resource may be specified but if they are files or
directories then they must be relative to the source directory that is
being built (the context of the build).

Each <src> may contain wildcards and matching will be done using Go’s
filepath.Match rules. For example:

ADD hom* /mydir/        # adds all files starting with "hom"
ADD hom?.txt /mydir/    # ? is replaced with any single character, e.g., "home.txt"

The <dest> is an absolute path, or a path relative to WORKDIR, into which
the source will be copied inside the destination container.

ADD test relativeDir/          # adds "test" to `WORKDIR`/relativeDir/
ADD test /absoluteDir/         # adds "test" to /absoluteDir/

All new files and directories are created with a UID and GID of 0.

In the case where <src> is a remote file URL, the destination will
have permissions of 600. If the remote file being retrieved has an HTTP
Last-Modified header, the timestamp from that header will be used
to set the mtime on the destination file. However, like any other file
processed during an ADD, mtime will not be included in the determination
of whether or not the file has changed and the cache should be updated.

Note:
If you build by passing a Dockerfile through STDIN (docker
build - < somefile), there is no build context, so the Dockerfile
can only contain a URL based ADD instruction. You can also pass a
compressed archive through STDIN: (docker build - < archive.tar.gz),
the Dockerfile at the root of the archive and the rest of the
archive will get used at the context of the build.

Note:
If your URL files are protected using authentication, you
will need to use RUN wget, RUN curl or use another tool from
within the container as the ADD instruction does not support
authentication.

Note:
The first encountered ADD instruction will invalidate the cache for all
following instructions from the Dockerfile if the contents of <src> have
changed. This includes invalidating the cache for RUN instructions.
See the Dockerfile Best Practices
guide for more information.

ADD obeys the following rules:

• The <src> path must be inside the context of the build;
  you cannot ADD ../something /something, because the first step of a
  docker build is to send the context directory (and subdirectories) to the
  docker daemon.

• If <src> is a URL and <dest> does not end with a trailing slash, then a
  file is downloaded from the URL and copied to <dest>.

• If <src> is a URL and <dest> does end with a trailing slash, then the
  filename is inferred from the URL and the file is downloaded to
  <dest>/<filename>. For instance, ADD http://example.com/foobar / would
  create the file /foobar. The URL must have a nontrivial path so that an
  appropriate filename can be discovered in this case (http://example.com
  will not work).

• If <src> is a directory, the entire contents of the directory are copied,
  including filesystem metadata.

Note:
The directory itself is not copied, just its contents.

• If <src> is a local tar archive in a recognized compression format
  (identity, gzip, bzip2 or xz) then it is unpacked as a directory. Resources
  from remote URLs are not decompressed. When a directory is copied or
  unpacked, it has the same behavior as tar -x: the result is the union of:

  1. Whatever existed at the destination path and
  2. The contents of the source tree, with conflicts resolved in favor
     of “2.” on a file-by-file basis.

  Note:
  Whether a file is identified as a recognized compression format or not
  is done solely based on the contents of the file, not the name of the file.
  For example, if an empty file happens to end with .tar.gz this will not
  be recognized as a compressed file and will not generate any kind of
  decompression error message, rather the file will simply be copied to the
  destination.

• If <src> is any other kind of file, it is copied individually along with
  its metadata. In this case, if <dest> ends with a trailing slash /, it
  will be considered a directory and the contents of <src> will be written
  at <dest>/base(<src>).

• If multiple <src> resources are specified, either directly or due to the
  use of a wildcard, then <dest> must be a directory, and it must end with
  a slash /.

• If <dest> does not end with a trailing slash, it will be considered a
  regular file and the contents of <src> will be written at <dest>.

• If <dest> doesn’t exist, it is created along with all missing directories
  in its path.

COPY

COPY has two forms:

• COPY <src>... <dest>
• COPY ["<src>",... "<dest>"] (this form is required for paths containing
  whitespace)

The COPY instruction copies new files or directories from <src>
and adds them to the filesystem of the container at the path <dest>.

Multiple <src> resource may be specified but they must be relative
to the source directory that is being built (the context of the build).

Each <src> may contain wildcards and matching will be done using Go’s
filepath.Match rules. For example:

COPY hom* /mydir/        # adds all files starting with "hom"
COPY hom?.txt /mydir/    # ? is replaced with any single character, e.g., "home.txt"

The <dest> is an absolute path, or a path relative to WORKDIR, into which
the source will be copied inside the destination container.

COPY test relativeDir/   # adds "test" to `WORKDIR`/relativeDir/
COPY test /absoluteDir/  # adds "test" to /absoluteDir/

All new files and directories are created with a UID and GID of 0.

Note:
If you build using STDIN (docker build - < somefile), there is no
build context, so COPY can’t be used.

COPY obeys the following rules:

• The <src> path must be inside the context of the build;
  you cannot COPY ../something /something, because the first step of a
  docker build is to send the context directory (and subdirectories) to the
  docker daemon.

• If <src> is a directory, the entire contents of the directory are copied,
  including filesystem metadata.

Note:
The directory itself is not copied, just its contents.

• If <src> is any other kind of file, it is copied individually along with
  its metadata. In this case, if <dest> ends with a trailing slash /, it
  will be considered a directory and the contents of <src> will be written
  at <dest>/base(<src>).

• If multiple <src> resources are specified, either directly or due to the
  use of a wildcard, then <dest> must be a directory, and it must end with
  a slash /.

• If <dest> does not end with a trailing slash, it will be considered a
  regular file and the contents of <src> will be written at <dest>.

• If <dest> doesn’t exist, it is created along with all missing directories
  in its path.

ENTRYPOINT

ENTRYPOINT has two forms:

• ENTRYPOINT ["executable", "param1", "param2"]
  (exec form, preferred)
• ENTRYPOINT command param1 param2
  (shell form)

An ENTRYPOINT allows you to configure a container that will run as an executable.

For example, the following will start nginx with its default content, listening
on port 80:

docker run -i -t --rm -p 80:80 nginx

Command line arguments to docker run <image> will be appended after all
elements in an exec form ENTRYPOINT, and will override all elements specified
using CMD.
This allows arguments to be passed to the entry point, i.e., docker run <image> -d
will pass the -d argument to the entry point.
You can override the ENTRYPOINT instruction using the docker run --entrypoint
flag.

The shell form prevents any CMD or run command line arguments from being
used, but has the disadvantage that your ENTRYPOINT will be started as a
subcommand of /bin/sh -c, which does not pass signals.
This means that the executable will not be the container’s PID 1 — and
will not receive Unix signals — so your executable will not receive a
SIGTERM from docker stop <container>.

Only the last ENTRYPOINT instruction in the Dockerfile will have an effect.

Exec form ENTRYPOINT example

You can use the exec form of ENTRYPOINT to set fairly stable default commands
and arguments and then use either form of CMD to set additional defaults that
are more likely to be changed.

FROM ubuntu
ENTRYPOINT ["top", "-b"]
CMD ["-c"]

When you run the container, you can see that top is the only process:

$ docker run -it --rm --name test  top -H
top - 08:25:00 up  7:27,  0 users,  load average: 0.00, 0.01, 0.05
Threads:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.1 us,  0.1 sy,  0.0 ni, 99.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   2056668 total,  1616832 used,   439836 free,    99352 buffers
KiB Swap:  1441840 total,        0 used,  1441840 free.  1324440 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
    1 root      20   0   19744   2336   2080 R  0.0  0.1   0:00.04 top

To examine the result further, you can use docker exec:

$ docker exec -it test ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  2.6  0.1  19752  2352 ?        Ss+  08:24   0:00 top -b -H
root         7  0.0  0.1  15572  2164 ?        R+   08:25   0:00 ps aux

And you can gracefully request top to shut down using docker stop test.

The following Dockerfile shows using the ENTRYPOINT to run Apache in the
foreground (i.e., as PID 1):

FROM debian:stable
RUN apt-get update && apt-get install -y --force-yes apache2
EXPOSE 80 443
VOLUME ["/var/www", "/var/log/apache2", "/etc/apache2"]
ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]

If you need to write a starter script for a single executable, you can ensure that
the final executable receives the Unix signals by using exec and gosu
commands:

#!/bin/bash
set -e

if [ "$1" = 'postgres' ]; then
    chown -R postgres "$PGDATA"

    if [ -z "$(ls -A "$PGDATA")" ]; then
        gosu postgres initdb
    fi

    exec gosu postgres "$@"
fi

exec "$@"

Lastly, if you need to do some extra cleanup (or communicate with other containers)
on shutdown, or are co-ordinating more than one executable, you may need to ensure
that the ENTRYPOINT script receives the Unix signals, passes them on, and then
does some more work:

#!/bin/sh
# Note: I've written this using sh so it works in the busybox container too

# USE the trap if you need to also do manual cleanup after the service is stopped,
#     or need to start multiple services in the one container
trap "echo TRAPed signal" HUP INT QUIT TERM

# start service in background here
/usr/sbin/apachectl start

echo "[hit enter key to exit] or run 'docker stop <container>'"
read

# stop service and clean up here
echo "stopping apache"
/usr/sbin/apachectl stop

echo "exited $0"

If you run this image with docker run -it --rm -p 80:80 --name test apache,
you can then examine the container’s processes with docker exec, or docker top,
and then ask the script to stop Apache:

$ docker exec -it test ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.0   4448   692 ?        Ss+  00:42   0:00 /bin/sh /run.sh 123 cmd cmd2
root        19  0.0  0.2  71304  4440 ?        Ss   00:42   0:00 /usr/sbin/apache2 -k start
www-data    20  0.2  0.2 360468  6004 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
www-data    21  0.2  0.2 360468  6000 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
root        81  0.0  0.1  15572  2140 ?        R+   00:44   0:00 ps aux
$ docker top test
PID                 USER                COMMAND
10035               root                {run.sh} /bin/sh /run.sh 123 cmd cmd2
10054               root                /usr/sbin/apache2 -k start
10055               33                  /usr/sbin/apache2 -k start
10056               33                  /usr/sbin/apache2 -k start
$ /usr/bin/time docker stop test
test
real	0m 0.27s
user	0m 0.03s
sys	0m 0.03s

Note: you can override the ENTRYPOINT setting using --entrypoint,
but this can only set the binary to exec (no sh -c will be used).

Note:
The exec form is parsed as a JSON array, which means that
you must use double-quotes (“) around words not single-quotes (‘).

Note:
Unlike the shell form, the exec form does not invoke a command shell.
This means that normal shell processing does not happen. For example,
ENTRYPOINT [ "echo", "$HOME" ] will not do variable substitution on $HOME.
If you want shell processing then either use the shell form or execute
a shell directly, for example: ENTRYPOINT [ "sh", "-c", "echo $HOME" ].
When using the exec form and executing a shell directly, as in the case for
the shell form, it is the shell that is doing the environment variable
expansion, not docker.

Shell form ENTRYPOINT example

You can specify a plain string for the ENTRYPOINT and it will execute in /bin/sh -c.
This form will use shell processing to substitute shell environment variables,
and will ignore any CMD or docker run command line arguments.
To ensure that docker stop will signal any long running ENTRYPOINT executable
correctly, you need to remember to start it with exec:

FROM ubuntu
ENTRYPOINT exec top -b

When you run this image, you’ll see the single PID 1 process:

$ docker run -it --rm --name test top
Mem: 1704520K used, 352148K free, 0K shrd, 0K buff, 140368121167873K cached
CPU:   5% usr   0% sys   0% nic  94% idle   0% io   0% irq   0% sirq
Load average: 0.08 0.03 0.05 2/98 6
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
    1     0 root     R     3164   0%   0% top -b

Which will exit cleanly on docker stop:

$ /usr/bin/time docker stop test
test
real	0m 0.20s
user	0m 0.02s
sys	0m 0.04s

If you forget to add exec to the beginning of your ENTRYPOINT:

FROM ubuntu
ENTRYPOINT top -b
CMD --ignored-param1

You can then run it (giving it a name for the next step):

$ docker run -it --name test top --ignored-param2
Mem: 1704184K used, 352484K free, 0K shrd, 0K buff, 140621524238337K cached
CPU:   9% usr   2% sys   0% nic  88% idle   0% io   0% irq   0% sirq
Load average: 0.01 0.02 0.05 2/101 7
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
    1     0 root     S     3168   0%   0% /bin/sh -c top -b cmd cmd2
    7     1 root     R     3164   0%   0% top -b

You can see from the output of top that the specified ENTRYPOINT is not PID 1.

If you then run docker stop test, the container will not exit cleanly — the
stop command will be forced to send a SIGKILL after the timeout:

$ docker exec -it test ps aux
PID   USER     COMMAND
    1 root     /bin/sh -c top -b cmd cmd2
    7 root     top -b
    8 root     ps aux
$ /usr/bin/time docker stop test
test
real	0m 10.19s
user	0m 0.04s
sys	0m 0.03s

Understand how CMD and ENTRYPOINT interact

Both CMD and ENTRYPOINT instructions define what command gets executed when running a container.
There are few rules that describe their co-operation.

1. Dockerfile should specify at least one of CMD or ENTRYPOINT commands.

2. ENTRYPOINT should be defined when using the container as an executable.

3. CMD should be used as a way of defining default arguments for an ENTRYPOINT command
   or for executing an ad-hoc command in a container.

4. CMD will be overridden when running the container with alternative arguments.

The table below shows what command is executed for different ENTRYPOINT / CMD combinations:

VOLUME

The VOLUME instruction creates a mount point with the specified name
and marks it as holding externally mounted volumes from native host or other
containers. The value can be a JSON array, VOLUME ["/var/log/"], or a plain
string with multiple arguments, such as VOLUME /var/log or VOLUME /var/log
/var/db. For more information/examples and mounting instructions via the
Docker client, refer to
Share Directories via Volumes
documentation.

The docker run command initializes the newly created volume with any data
that exists at the specified location within the base image. For example,
consider the following Dockerfile snippet:

FROM ubuntu
RUN mkdir /myvol
RUN echo "hello world" > /myvol/greeting
VOLUME /myvol

This Dockerfile results in an image that causes docker run, to
create a new mount point at /myvol and copy the greeting file
into the newly created volume.

Note:
If any build steps change the data within the volume after it has been
declared, those changes will be discarded.

Note:
The list is parsed as a JSON array, which means that
you must use double-quotes (“) around words not single-quotes (‘).

USER

The USER instruction sets the user name or UID to use when running the image
and for any RUN, CMD and ENTRYPOINT instructions that follow it in the
Dockerfile.

WORKDIR

The WORKDIR instruction sets the working directory for any RUN, CMD,
ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile.
If the WORKDIR doesn’t exist, it will be created even if it’s not used in any
subsequent Dockerfile instruction.

It can be used multiple times in the one Dockerfile. If a relative path
is provided, it will be relative to the path of the previous WORKDIR
instruction. For example:

WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd

The output of the final pwd command in this Dockerfile would be
/a/b/c.

The WORKDIR instruction can resolve environment variables previously set using
ENV. You can only use environment variables explicitly set in the Dockerfile.
For example:

ENV DIRPATH /path
WORKDIR $DIRPATH/$DIRNAME
RUN pwd

The output of the final pwd command in this Dockerfile would be
/path/$DIRNAME

ARG

ARG <name>[=<default value>]

The ARG instruction defines a variable that users can pass at build-time to
the builder with the docker build command using the
--build-arg <varname>=<value> flag. If a user specifies a build argument
that was not defined in the Dockerfile, the build outputs an error.

One or more build-args were not consumed, failing build.

The Dockerfile author can define a single variable by specifying ARG once or many
variables by specifying ARG more than once. For example, a valid Dockerfile:

FROM busybox
ARG user1
ARG buildno
...

A Dockerfile author may optionally specify a default value for an ARG instruction:

FROM busybox
ARG user1=someuser
ARG buildno=1
...

If an ARG value has a default and if there is no value passed at build-time, the
builder uses the default.

An ARG variable definition comes into effect from the line on which it is
defined in the Dockerfile not from the argument’s use on the command-line or
elsewhere. For example, consider this Dockerfile:

1 FROM busybox
2 USER ${user:-some_user}
3 ARG user
4 USER $user
...

A user builds this file by calling:

$ docker build --build-arg user=what_user Dockerfile

The USER at line 2 evaluates to some_user as the user variable is defined on the
subsequent line 3. The USER at line 4 evaluates to what_user as user is
defined and the what_user value was passed on the command line. Prior to its definition by an
ARG instruction, any use of a variable results in an empty string.

Warning: It is not recommended to use build-time variables for
passing secrets like github keys, user credentials etc. Build-time variable
values are visible to any user of the image with the docker history command.

You can use an ARG or an ENV instruction to specify variables that are
available to the RUN instruction. Environment variables defined using the
ENV instruction always override an ARG instruction of the same name. Consider
this Dockerfile with an ENV and ARG instruction.

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER v1.0.0
4 RUN echo $CONT_IMG_VER

Then, assume this image is built with this command:

$ docker build --build-arg CONT_IMG_VER=v2.0.1 Dockerfile

In this case, the RUN instruction uses v1.0.0 instead of the ARG setting
passed by the user:v2.0.1 This behavior is similar to a shell
script where a locally scoped variable overrides the variables passed as
arguments or inherited from environment, from its point of definition.

Using the example above but a different ENV specification you can create more
useful interactions between ARG and ENV instructions:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER ${CONT_IMG_VER:-v1.0.0}
4 RUN echo $CONT_IMG_VER

Unlike an ARG instruction, ENV values are always persisted in the built
image. Consider a docker build without the –build-arg flag:

$ docker build Dockerfile

Using this Dockerfile example, CONT_IMG_VER is still persisted in the image but
its value would be v1.0.0 as it is the default set in line 3 by the ENV instruction.

The variable expansion technique in this example allows you to pass arguments
from the command line and persist them in the final image by leveraging the
ENV instruction. Variable expansion is only supported for a limited set of
Dockerfile instructions.

Docker has a set of predefined ARG variables that you can use without a
corresponding ARG instruction in the Dockerfile.

• HTTP_PROXY
• http_proxy
• HTTPS_PROXY
• https_proxy
• FTP_PROXY
• ftp_proxy
• NO_PROXY
• no_proxy

To use these, simply pass them on the command line using the flag:

--build-arg <varname>=<value>

Impact on build caching

ARG variables are not persisted into the built image as ENV variables are.
However, ARG variables do impact the build cache in similar ways. If a
Dockerfile defines an ARG variable whose value is different from a previous
build, then a “cache miss” occurs upon its first usage, not its definition. In
particular, all RUN instructions following an ARG instruction use the ARG
variable implicitly (as an environment variable), thus can cause a cache miss.

For example, consider these two Dockerfile:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo $CONT_IMG_VER

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo hello

If you specify --build-arg CONT_IMG_VER=<value> on the command line, in both
cases, the specification on line 2 does not cause a cache miss; line 3 does
cause a cache miss.ARG CONT_IMG_VER causes the RUN line to be identified
as the same as running CONT_IMG_VER=<value> echo hello, so if the <value>
changes, we get a cache miss.

Consider another example under the same command line:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER $CONT_IMG_VER
4 RUN echo $CONT_IMG_VER

In this example, the cache miss occurs on line 3. The miss happens because
the variable’s value in the ENV references the ARG variable and that
variable is changed through the command line. In this example, the ENV
command causes the image to include the value.

If an ENV instruction overrides an ARG instruction of the same name, like
this Dockerfile:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER hello
4 RUN echo $CONT_IMG_VER

Line 3 does not cause a cache miss because the value of CONT_IMG_VER is a
constant (hello). As a result, the environment variables and values used on
the RUN (line 4) doesn’t change between builds.

ONBUILD

The ONBUILD instruction adds to the image a trigger instruction to
be executed at a later time, when the image is used as the base for
another build. The trigger will be executed in the context of the
downstream build, as if it had been inserted immediately after the
FROM instruction in the downstream Dockerfile.

Any build instruction can be registered as a trigger.

This is useful if you are building an image which will be used as a base
to build other images, for example an application build environment or a
daemon which may be customized with user-specific configuration.

For example, if your image is a reusable Python application builder, it
will require application source code to be added in a particular
directory, and it might require a build script to be called after
that. You can’t just call ADD and RUN now, because you don’t yet
have access to the application source code, and it will be different for
each application build. You could simply provide application developers
with a boilerplate Dockerfile to copy-paste into their application, but
that is inefficient, error-prone and difficult to update because it
mixes with application-specific code.

The solution is to use ONBUILD to register advance instructions to
run later, during the next build stage.

Here’s how it works:

1. When it encounters an ONBUILD instruction, the builder adds a
   trigger to the metadata of the image being built. The instruction
   does not otherwise affect the current build.
2. At the end of the build, a list of all triggers is stored in the
   image manifest, under the key OnBuild. They can be inspected with
   the docker inspect command.
3. Later the image may be used as a base for a new build, using the
   FROM instruction. As part of processing the FROM instruction,
   the downstream builder looks for ONBUILD triggers, and executes
   them in the same order they were registered. If any of the triggers
   fail, the FROM instruction is aborted which in turn causes the
   build to fail. If all triggers succeed, the FROM instruction
   completes and the build continues as usual.
4. Triggers are cleared from the final image after being executed. In
   other words they are not inherited by “grand-children” builds.

For example you might add something like this:

[...]
ONBUILD ADD . /app/src
ONBUILD RUN /usr/local/bin/python-build --dir /app/src
[...]

Warning: Chaining ONBUILD instructions using ONBUILD ONBUILD isn’t allowed.

Warning: The ONBUILD instruction may not trigger FROM or MAINTAINER instructions.

STOPSIGNAL

The STOPSIGNAL instruction sets the system call signal that will be sent to the container to exit.
This signal can be a valid unsigned number that matches a position in the kernel’s syscall table, for instance 9,
or a signal name in the format SIGNAME, for instance SIGKILL.

HEALTHCHECK

The HEALTHCHECK instruction has two forms:

• HEALTHCHECK [OPTIONS] CMD command (check container health by running a command inside the container)
• HEALTHCHECK NONE (disable any healthcheck inherited from the base image)

The HEALTHCHECK instruction tells Docker how to test a container to check that
it is still working. This can detect cases such as a web server that is stuck in
an infinite loop and unable to handle new connections, even though the server
process is still running.

When a container has a healthcheck specified, it has a health status in
addition to its normal status. This status is initially starting. Whenever a
health check passes, it becomes healthy (whatever state it was previously in).
After a certain number of consecutive failures, it becomes unhealthy.

The options that can appear before CMD are:

• --interval=DURATION (default: 30s)
• --timeout=DURATION (default: 30s)
• --retries=N (default: 3)

The health check will first run interval seconds after the container is
started, and then again interval seconds after each previous check completes.

If a single run of the check takes longer than timeout seconds then the check
is considered to have failed.

It takes retries consecutive failures of the health check for the container
to be considered unhealthy.

There can only be one HEALTHCHECK instruction in a Dockerfile. If you list
more than one then only the last HEALTHCHECK will take effect.

The command after the CMD keyword can be either a shell command (e.g. HEALTHCHECK
CMD /bin/check-running) or an exec array (as with other Dockerfile commands;
see e.g. ENTRYPOINT for details).

The command’s exit status indicates the health status of the container.
The possible values are:

• 0: success — the container is healthy and ready for use
• 1: unhealthy — the container is not working correctly
• 2: reserved — do not use this exit code

For example, to check every five minutes or so that a web-server is able to
serve the site’s main page within three seconds:

HEALTHCHECK --interval=5m --timeout=3s \
  CMD curl -f http://localhost/ || exit 1

To help debug failing probes, any output text (UTF-8 encoded) that the command writes
on stdout or stderr will be stored in the health status and can be queried with
docker inspect. Such output should be kept short (only the first 4096 bytes
are stored currently).

When the health status of a container changes, a health_status event is
generated with the new status.

The HEALTHCHECK feature was added in Docker 1.12.

SHELL

SHELL ["executable", "parameters"]

The SHELL instruction allows the default shell used for the shell form of
commands to be overridden. The default shell on Linux is ["/bin/sh", "-c"], and on
Windows is ["cmd", "/S", "/C"]. The SHELL instruction must be written in JSON
form in a Dockerfile.

The SHELL instruction is particularly useful on Windows where there are
two commonly used and quite different native shells: cmd and powershell, as
well as alternate shells available including sh.

The SHELL instruction can appear multiple times. Each SHELL instruction overrides
all previous SHELL instructions, and affects all subsequent instructions. For example:

FROM windowsservercore

# Executed as cmd /S /C echo default
RUN echo default

# Executed as cmd /S /C powershell -command Write-Host default
RUN powershell -command Write-Host default

# Executed as powershell -command Write-Host hello
SHELL ["powershell", "-command"]
RUN Write-Host hello

# Executed as cmd /S /C echo hello
SHELL ["cmd", "/S"", "/C"]
RUN echo hello

The following instructions can be affected by the SHELL instruction when the
shell form of them is used in a Dockerfile: RUN, CMD and ENTRYPOINT.

The following example is a common pattern found on Windows which can be
streamlined by using the SHELL instruction:

...
RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
...

The command invoked by docker will be:

cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"

This is inefficient for two reasons. First, there is an un-necessary cmd.exe command
processor (aka shell) being invoked. Second, each RUN instruction in the shell
form requires an extra powershell -command prefixing the command.

To make this more efficient, one of two mechanisms can be employed. One is to
use the JSON form of the RUN command such as:

...
RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""]
...

While the JSON form is unambiguous and does not use the un-necessary cmd.exe,
it does require more verbosity through double-quoting and escaping. The alternate
mechanism is to use the SHELL instruction and the shell form,
making a more natural syntax for Windows users, especially when combined with
the escape parser directive:

# escape=`

FROM windowsservercore
SHELL ["powershell","-command"]
RUN New-Item -ItemType Directory C:\Example
ADD Execute-MyCmdlet.ps1 c:\example\
RUN c:\example\Execute-MyCmdlet -sample 'hello world'

Resulting in:

PS E:\docker\build\shell> docker build -t shell .
Sending build context to Docker daemon 3.584 kB
Step 1 : FROM windowsservercore
 ---> 5bc36a335344
Step 2 : SHELL powershell -command
 ---> Running in 87d7a64c9751
 ---> 4327358436c1
Removing intermediate container 87d7a64c9751
Step 3 : RUN New-Item -ItemType Directory C:\Example
 ---> Running in 3e6ba16b8df9


    Directory: C:\


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
d-----         6/2/2016   2:59 PM                Example


 ---> 1f1dfdcec085
Removing intermediate container 3e6ba16b8df9
Step 4 : ADD Execute-MyCmdlet.ps1 c:\example\
 ---> 6770b4c17f29
Removing intermediate container b139e34291dc
Step 5 : RUN c:\example\Execute-MyCmdlet -sample 'hello world'
 ---> Running in abdcf50dfd1f
Hello from Execute-MyCmdlet.ps1 - passed hello world
 ---> ba0e25255fda
Removing intermediate container abdcf50dfd1f
Successfully built ba0e25255fda
PS E:\docker\build\shell>

The SHELL instruction could also be used to modify the way in which
a shell operates. For example, using SHELL cmd /S /C /V:ON|OFF on Windows, delayed
environment variable expansion semantics could be modified.

The SHELL instruction can also be used on Linux should an alternate shell be
required such zsh, csh, tcsh and others.

The SHELL feature was added in Docker 1.12.

Dockerfile examples

Below you can see some examples of Dockerfile syntax. If you’re interested in
something more realistic, take a look at the list of Dockerization examples.

# Nginx
#
# VERSION               0.0.1

FROM      ubuntu
MAINTAINER Victor Vieux <victor@docker.com>

LABEL Description="This image is used to start the foobar executable" Vendor="ACME Products" Version="1.0"
RUN apt-get update && apt-get install -y inotify-tools nginx apache2 openssh-server

# Firefox over VNC
#
# VERSION               0.3

FROM ubuntu

# Install vnc, xvfb in order to create a 'fake' display and firefox
RUN apt-get update && apt-get install -y x11vnc xvfb firefox
RUN mkdir ~/.vnc
# Setup a password
RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
# Autostart firefox (might not be the best way, but it does the trick)
RUN bash -c 'echo "firefox" >> /.bashrc'

EXPOSE 5900
CMD    ["x11vnc", "-forever", "-usepw", "-create"]

# Multiple images example
#
# VERSION               0.1

FROM ubuntu
RUN echo foo > bar
# Will output something like ===> 907ad6c2736f

FROM ubuntu
RUN echo moo > oink
# Will output something like ===> 695d7793cbe4

# You᾿ll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
# /oink.