DOCKERFILE(5) May 2014 DOCKERFILE(5)
NAME
Dockerfile - automate the steps of creating a Docker image
INTRODUCTION
The Dockerfile is a configuration file that automates the steps of
creating a Docker image. It is similar to a Makefile. Docker reads
instructions from the Dockerfile to automate the steps otherwise
performed manually to create an image. To build an image, create a file
called Dockerfile.
The Dockerfile describes the steps taken to assemble the image. When
the Dockerfile has been created, call the docker build command, using
the path of directory that contains Dockerfile as the argument.
SYNOPSIS
INSTRUCTION arguments
For example:
FROM image
DESCRIPTION
A Dockerfile is a file that automates the steps of creating a Docker
image. A Dockerfile is similar to a Makefile.
USAGE
docker build .
-- Runs the steps and commits them, building a final image.
The path to the source repository defines where to find the context
of the
build. The build is run by the Docker daemon, not the CLI. The whole
context must be transferred to the daemon. The Docker CLI reports
"Sending build context to Docker daemon" when the context is sent to
the
daemon.
docker build -t repository/tag .
-- specifies a repository and tag at which to save the new image if the
build
succeeds. The Docker daemon runs the steps one-by-one, committing the
result
to a new image if necessary, before finally outputting the ID of the
new
image. The Docker daemon automatically cleans up the context it is
given.
Docker re-uses intermediate images whenever possible. This
significantly
accelerates the docker build process.
FORMAT
FROM image
FROM image:tag
FROM image@digest
-- The FROM instruction sets the base image for subsequent
instructions. A
valid Dockerfile must have FROM as its first instruction. The image
can be any
valid image. It is easy to start by pulling an image from the public
repositories.
-- FROM must be the first non-comment instruction in Dockerfile.
-- FROM may appear multiple times within a single Dockerfile in order
to create
multiple images. Make a note of the last image ID output by the
commit before
each new FROM command.
-- If no tag is given to the FROM instruction, Docker applies the
latest tag. If the used tag does not exist, an error is returned.
-- If no digest is given to the FROM instruction, Docker applies the
latest tag. If the used tag does not exist, an error is returned.
MAINTAINER
-- MAINTAINER sets the Author field for the generated images.
Useful for providing users with an email or url for support.
RUN
-- RUN has two forms:
# the command is run in a shell - /bin/sh -c
RUN <command>
# Executable form
RUN ["executable", "param1", "param2"]
-- The RUN instruction executes any commands in a new layer on top of
the current
image and commits the results. The committed image is used for the
next step in
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 the history of an image. This is similar to source
control. The
exec form makes it possible to avoid shell string munging. The exec
form makes
it possible to RUN commands using a base image that does not contain
/bin/sh.
Note that the exec form is parsed as a JSON array, which means that you
must
use double-quotes (") around words not single-quotes (').
CMD
-- CMD has three forms:
# Executable form
CMD ["executable", "param1", "param2"]`
# Provide default arguments to ENTRYPOINT
CMD ["param1", "param2"]`
# the command is run in a shell - /bin/sh -c
CMD command param1 param2
-- There should be only one CMD in a Dockerfile. If more than one CMD
is listed, only
the last CMD takes effect.
The main purpose of a CMD is to provide defaults for an executing
container.
These defaults may include an executable, or they can omit the
executable. If
they omit the executable, an ENTRYPOINT must be specified.
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, the <command> executes in
/bin/sh -c:
Note that the exec form is parsed as a JSON array, which means that you
must
use double-quotes (") around words not single-quotes (').
FROM ubuntu
CMD echo "This is a test." | wc -
-- If you run 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 form of CMD. All additional parameters must be individually
expressed
as strings in the array:
FROM ubuntu
CMD ["/usr/bin/wc","--help"]
-- To make the container run the same executable every time, use
ENTRYPOINT in
combination with CMD.
If the user specifies arguments to docker run, the specified commands
override the default in CMD.
Do not confuse RUN with CMD. RUN runs a command and commits the
result.
CMD executes nothing at build time, but specifies the intended
command for
the image.
LABEL
-- LABEL <key>=<value> [<key>=<value> ...]or
LABEL <key>[ <value>]
LABEL <key>[ <value>]
...
The LABEL instruction adds metadata to an image. A LABEL is a
key-value pair. To specify a LABEL without a value, simply use an
empty
string. To include spaces within a LABEL value, use quotes and
backslashes as you would in command-line parsing.
LABEL com.example.vendor="ACME Incorporated"
LABEL com.example.vendor "ACME Incorporated"
LABEL com.example.vendor.is-beta ""
LABEL com.example.vendor.is-beta=
LABEL com.example.vendor.is-beta=""
An image can have more than one label. To specify multiple labels,
separate
each key-value pair by a space.
Labels are additive including LABELs in FROM images. As the system
encounters and then applies a new label, new keys override any
previous
labels with identical keys.
To display an image's labels, use the docker inspect command.
EXPOSE
-- EXPOSE <port> [<port>...]
The EXPOSE instruction informs Docker that the container listens on
the
specified network ports at runtime. Docker uses this information to
interconnect containers using links and to set up port redirection on
the host
system.
ENV
-- ENV <key> <value>
The ENV instruction sets the environment variable <key> to
the value <value>. This value is passed to all future
RUN, ENTRYPOINT, and CMD instructions. This is
functionally equivalent to prefixing the command with <key>=<value>.
The
environment variables that are set with ENV persist when a container
is run
from the resulting image. Use docker inspect to inspect these values,
and
change them using docker run --env <key>=<value>.
Note that setting "ENV DEBIAN_FRONTEND noninteractive" may cause
unintended consequences, because it will persist when the container
is run
interactively, as with the following command: docker run -t -i image
bash
ADD
-- ADD has two forms:
ADD <src> <dest>
# Required for paths with whitespace
ADD ["<src>",... "<dest>"]
The ADD instruction copies new files, directories
or remote file URLs to the filesystem of the container at path
<dest>.
Multiple <src> resources 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). The <dest> is the absolute path, or path
relative
to WORKDIR, into which the source is copied inside the target
container.
If the <src> argument is a local file in a recognized compression
format
(tar, gzip, bzip2, etc) then it is unpacked at the specified <dest>
in the
container's filesystem. Note that only local compressed files will
be unpacked,
i.e., the URL download and archive unpacking features cannot be used
together.
All new directories are created with mode 0755 and with the uid and
gid of 0.
COPY
-- COPY has two forms:
COPY <src> <dest>
# Required for paths with whitespace
COPY ["<src>",... "<dest>"]
The COPY instruction copies new files from <src> and
adds them to the filesystem of the container at path <dest>. The
<src> must be
the path to a file or directory relative to the source directory that
is
being built (the context of the build) or a remote file URL. The
<dest> is an
absolute path, or a path relative to WORKDIR, into which the source
will
be copied inside the target container. If you COPY an archive file it
will
land in the container exactly as it appears in the build context
without any
attempt to unpack it. All new files and directories are created with
mode 0755
and with the uid and gid of 0.
ENTRYPOINT
-- ENTRYPOINT has two forms:
# executable form
ENTRYPOINT ["executable", "param1", "param2"]`
# run command in a shell - /bin/sh -c
ENTRYPOINT command param1 param2
-- An ENTRYPOINT helps you configure a
container that can be run as an executable. When you specify an
ENTRYPOINT,
the whole container runs as if it was only that executable. The
ENTRYPOINT
instruction adds an entry command that is not overwritten when
arguments are
passed to docker run. This is different from the behavior of CMD.
This allows
arguments to be passed to the entrypoint, for instance docker run
<image> -d
passes the -d argument to the ENTRYPOINT. Specify parameters either
in the
ENTRYPOINT JSON array (as in the preferred exec form above), or by
using a CMD
statement. Parameters in the ENTRYPOINT are not overwritten by the
docker run
arguments. Parameters specified via CMD are overwritten by docker
run
arguments. Specify a plain string for the ENTRYPOINT, and it will
execute in
/bin/sh -c, like a CMD instruction:
FROM ubuntu
ENTRYPOINT wc -l -
This means that the Dockerfile's image always takes stdin as input
(that's
what "-" means), and prints the number of lines (that's what "-l"
means). To
make this optional but default, use a CMD:
FROM ubuntu
CMD ["-l", "-"]
ENTRYPOINT ["/usr/bin/wc"]
VOLUME
-- VOLUME ["/data"]
The VOLUME instruction creates a mount point with the specified name
and marks
it as holding externally-mounted volumes from the native host or from
other
containers.
USER
-- USER daemon
Sets the username or UID used for running subsequent commands.
The USER instruction can optionally be used to set the group or GID.
The
followings examples are all valid:
USER [user | user:group | uid | uid:gid | user:gid | uid:group ]
Until the USER instruction is set, instructions will be run as root.
The USER
instruction can be used any number of times in a Dockerfile, and will
only affect
subsequent commands.
WORKDIR
-- WORKDIR /path/to/workdir
The WORKDIR instruction sets the working directory for the RUN, CMD,
ENTRYPOINT, COPY and ADD Dockerfile commands that follow it. It can
be used multiple times in a single Dockerfile. Relative paths are
defined
relative to the path of the previous WORKDIR instruction. For
example:
WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd
In the above example, the output of the pwd command is a/b/c.
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 a warning.
[Warning] One or more build-args [foo] were not consumed
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. <#environment-replacement>
Docker has a set of predefined ARG variables that you can use without a
corresponding ARG instruction in the Dockerfile.
o HTTP_PROXY
o http_proxy
o HTTPS_PROXY
o https_proxy
o FTP_PROXY
o ftp_proxy
o NO_PROXY
o no_proxy
To use these, simply pass them on the command line using the
--build-arg
<varname>=<value> flag.
ONBUILD
-- ONBUILD [INSTRUCTION]
The ONBUILD instruction adds a trigger instruction to an image. The
trigger is executed at a later time, when the image is used as the
base for
another build. Docker executes the trigger in the context of the
downstream
build, as if the trigger existed immediately after the FROM
instruction in
the downstream Dockerfile.
You can register any build instruction as a trigger. A trigger is
useful if
you are defining an image to use as a base for building other images.
For
example, if you are defining an application build environment or a
daemon that
is customized with a user-specific configuration.
Consider an image intended as a reusable python application builder. It
must
add application source code to a particular directory, and might need
a build
script 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 is
different
for each application build.
-- Providing application developers with a boilerplate Dockerfile to
copy-paste
into their application is inefficient, error-prone, and
difficult to update because it mixes with application-specific code.
The solution is to use ONBUILD to register instructions in advance,
to
run later, during the next build stage.
HISTORY
*May 2014, Compiled by Zac Dover (zdover at redhat dot com) based on
docker.com Dockerfile documentation. *Feb 2015, updated by Brian Goff
(cpuguy83@gmail.com) for readability *Sept 2015, updated by Sally
O'Malley (somalley@redhat.com) *Oct 2016, updated by Addam Hardy
(addam.hardy@gmail.com)
Zac Dover Docker User Manuals DOCKERFILE(5)