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Hints, tips and guidelines for writing clean, reliable Dockerfiles |
parent image, images, dockerfile, best practices, hub, official repo |
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Best practices for writing Dockerfiles |
Docker can build images automatically by reading the instructions from a
Dockerfile, a text file that contains all the commands, in order, needed to
build a given image. Dockerfiles adhere to a specific format and use a
specific set of instructions. You can learn the basics on the
Dockerfile Reference page. If
you’re new to writing Dockerfiles, you should start there.
This document covers the best practices and methods recommended by Docker, Inc. and the Docker community for building efficient images. To see many of these practices and recommendations in action, check out the Dockerfile for buildpack-deps.
Note: for more detailed explanations of any of the Dockerfile commands mentioned here, visit the Dockerfile Reference page.
The container produced by the image your Dockerfile defines should be as
ephemeral as possible. By “ephemeral,” we mean that it can be stopped and
destroyed and a new one built and put in place with an absolute minimum of
set-up and configuration. You may want to take a look at the
Processes section of the 12 Factor app
methodology to get a feel for the motivations of running containers in such a
stateless fashion.
When you issue a docker build command, the current working directory is called
the build context. By default, the Dockerfile is assumed to be located here,
but you can specify a different location with the file flag (-f). Regardless
of where the Dockerfile actually lives, all of the recursive contents of files
and directories in the current directory are sent to the Docker daemon as the
build context.
Build context example
Create a directory for the build context and
cdinto it. Write "hello" into a text file namedhelloand create a Dockerfile that runscaton it. Build the image from within the build context (.):mkdir myproject && cd myproject echo "hello" > hello echo -e "FROM busybox\nCOPY /hello /\nRUN cat /hello" > Dockerfile docker build -t helloapp:v1 .Now move
Dockerfileandhellointo separate directories and build a second version of the image (without relying on cache from the last build). Use the-fto point to the Dockerfile and specify the directory of the build context:mkdir -p dockerfiles context mv Dockerfile dockerfiles && mv hello context docker build --no-cache -t helloapp:v2 -f dockerfiles/Dockerfile context
Inadvertently including files that are not necessary for building an image
results in a larger build context and larger image size. This can increase build
time, time to pull and push the image, and the runtime size of containers. To
see how big your build context is, look for a message like this when building
your Dockerfile:
Sending build context to Docker daemon 187.8MB
To exclude files which are not relevant to the build, without restructuring your
source repository, use a .dockerignore file. This file supports
exclusion patterns similar to .gitignore files. For information on creating
one, see the .dockerignore file.
In addition to using a .dockerignore file, check out the information below
on multi-stage builds.
If you use Docker 17.05 or higher, you can use multi-stage builds to drastically reduce the size of your final image, without the need to jump through hoops to reduce the number of intermediate layers or remove intermediate files during the build.
Images being built by the final stage only, you can most of the time benefit both the build cache and minimize images layers.
Your build stage may contain several layers, ordered from the less frequently changed to the more frequently changed for example:
-
Install tools you need to build your application
-
Install or update library dependencies
-
Generate your application
A Dockerfile for a go application could look like:
FROM golang:1.9.2-alpine3.6 AS build
# Install tools required to build the project
# We need to run `docker build --no-cache .` to update those dependencies
RUN apk add --no-cache git
RUN go get github.com/golang/dep/cmd/dep
# Gopkg.toml and Gopkg.lock lists project dependencies
# These layers are only re-built when Gopkg files are updated
COPY Gopkg.lock Gopkg.toml /go/src/project/
WORKDIR /go/src/project/
# Install library dependencies
RUN dep ensure -vendor-only
# Copy all project and build it
# This layer is rebuilt when ever a file has changed in the project directory
COPY . /go/src/project/
RUN go build -o /bin/project
# This results in a single layer image
FROM scratch
COPY --from=build /bin/project /bin/project
ENTRYPOINT ["/bin/project"]
CMD ["--help"]
To reduce complexity, dependencies, file sizes, and build times, you should avoid installing extra or unnecessary packages just because they might be “nice to have.” For example, you don’t need to include a text editor in a database image.
Decoupling applications into multiple containers makes it much easier to scale horizontally and reuse containers. For instance, a web application stack might consist of three separate containers, each with its own unique image, to manage the web application, database, and an in-memory cache in a decoupled manner.
You may have heard that there should be "one process per container". While this mantra has good intentions, it is not necessarily true that there should be only one operating system process per container. In addition to the fact that containers can now be spawned with an init process, some programs might spawn additional processes of their own accord. For instance, Celery can spawn multiple worker processes, or Apache might create a process per request. While "one process per container" is frequently a good rule of thumb, it is not a hard and fast rule. Use your best judgment to keep containers as clean and modular as possible.
If containers depend on each other, you can use Docker container networks to ensure that these containers can communicate.
Prior to Docker 17.05, and even more, prior to Docker 1.10, it was important to minimize the number of layers in your image. The following improvements have mitigated this need:
-
In Docker 1.10 and higher, only
RUN,COPY, andADDinstructions create layers. Other instructions create temporary intermediate images, and no longer directly increase the size of the build. -
Docker 17.05 and higher add support for multi-stage builds, which allow you to copy only the artifacts you need into the final image. This allows you to include tools and debug information in your intermediate build stages without increasing the size of the final image.
Whenever possible, ease later changes by sorting multi-line arguments
alphanumerically. This helps you avoid duplication of packages and make the
list much easier to update. This also makes PRs a lot easier to read and
review. Adding a space before a backslash (\) helps as well.
Here’s an example from the buildpack-deps image:
RUN apt-get update && apt-get install -y \
bzr \
cvs \
git \
mercurial \
subversion
During the process of building an image Docker steps through the
instructions in your Dockerfile executing each in the order specified.
As each instruction is examined Docker looks for an existing image in its
cache that it can reuse, rather than creating a new (duplicate) image.
If you do not want to use the cache at all you can use the --no-cache=true
option on the docker build command.
However, if you do let Docker use its cache then it is very important to understand when it can, and cannot, find a matching image. The basic rules that Docker follows are outlined below:
-
Starting with a parent image that is already in the cache, the next instruction is compared against all child images derived from that base image to see if one of them was built using the exact same instruction. If not, the cache is invalidated.
-
In most cases simply comparing the instruction in the
Dockerfilewith one of the child images is sufficient. However, certain instructions require a little more examination and explanation. -
For the
ADDandCOPYinstructions, the contents of the file(s) in the image are examined and a checksum is calculated for each file. The last-modified and last-accessed times of the file(s) are not considered in these checksums. During the cache lookup, the checksum is compared against the checksum in the existing images. If anything has changed in the file(s), such as the contents and metadata, then the cache is invalidated. -
Aside from the
ADDandCOPYcommands, cache checking does not look at the files in the container to determine a cache match. For example, when processing aRUN apt-get -y updatecommand the files updated in the container are not examined to determine if a cache hit exists. In that case just the command string itself is used to find a match.
Once the cache is invalidated, all subsequent Dockerfile commands
generate new images and the cache is not used.
These recommendations help you to write an efficient and maintainable
Dockerfile.
Dockerfile reference for the FROM instruction
Whenever possible, use current Official Repositories as the basis for your image. We recommend the Alpine image since it’s very tightly controlled and kept minimal (currently under 5 mb), while still being a full distribution.
You can add labels to your image to help organize images by project, record
licensing information, to aid in automation, or for other reasons. For each
label, add a line beginning with LABEL and with one or more key-value pairs.
The following examples show the different acceptable formats. Explanatory comments are included inline.
Note: If your string contains spaces, it must be quoted or the spaces must be escaped. If your string contains inner quote characters (
"), escape them as well.
# Set one or more individual labels
LABEL com.example.version="0.0.1-beta"
LABEL vendor="ACME Incorporated"
LABEL com.example.release-date="2015-02-12"
LABEL com.example.version.is-production=""
An image can have more than one label. Prior to Docker 1.10, it was recommended
to combine all labels into a single LABEL instruction, to prevent extra layers
from being created. This is no longer necessary, but combining labels is still
supported.
# Set multiple labels on one line
LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"
The above can also be written as:
# Set multiple labels at once, using line-continuation characters to break long lines
LABEL vendor=ACME\ Incorporated \
com.example.is-beta= \
com.example.is-production="" \
com.example.version="0.0.1-beta" \
com.example.release-date="2015-02-12"
See Understanding object labels for guidelines about acceptable label keys and values. For information about querying labels, refer to the items related to filtering in Managing labels on objects. See also LABEL in the Dockerfile reference.
Dockerfile reference for the RUN instruction
As always, to make your Dockerfile more readable, understandable, and
maintainable, split long or complex RUN statements on multiple lines separated
with backslashes.
Probably the most common use-case for RUN is an application of apt-get. The
RUN apt-get command, because it installs packages, has several gotchas to look
out for.
You should avoid RUN apt-get upgrade or dist-upgrade, as many of the
“essential” packages from the parent images can't upgrade inside an
unprivileged container.
If a package contained in the parent image is out-of-date, you should contact its
maintainers. If you know there’s a particular package, foo, that needs to be updated, use
apt-get install -y foo to update automatically.
Always combine RUN apt-get update with apt-get install in the same RUN
statement. For example:
RUN apt-get update && apt-get install -y \
package-bar \
package-baz \
package-foo
Using apt-get update alone in a RUN statement causes caching issues and
subsequent apt-get install instructions fail.
For example, say you have a Dockerfile:
FROM ubuntu:14.04
RUN apt-get update
RUN apt-get install -y curl
After building the image, all layers are in the Docker cache. Suppose you later
modify apt-get install by adding extra package:
FROM ubuntu:14.04
RUN apt-get update
RUN apt-get install -y curl nginx
Docker sees the initial and modified instructions as identical and reuses the
cache from previous steps. As a result the apt-get update is NOT executed
because the build uses the cached version. Because the apt-get update is not
run, your build can potentially get an outdated version of the curl and nginx
packages.
Using RUN apt-get update && apt-get install -y ensures your Dockerfile
installs the latest package versions with no further coding or manual
intervention. This technique is known as "cache busting". You can also achieve
cache-busting by specifying a package version. This is known as version pinning,
for example:
RUN apt-get update && apt-get install -y \
package-bar \
package-baz \
package-foo=1.3.*
Version pinning forces the build to retrieve a particular version regardless of what’s in the cache. This technique can also reduce failures due to unanticipated changes in required packages.
Below is a well-formed RUN instruction that demonstrates all the apt-get
recommendations.
RUN apt-get update && apt-get install -y \
aufs-tools \
automake \
build-essential \
curl \
dpkg-sig \
libcap-dev \
libsqlite3-dev \
mercurial \
reprepro \
ruby1.9.1 \
ruby1.9.1-dev \
s3cmd=1.1.* \
&& rm -rf /var/lib/apt/lists/*
The s3cmd instructions specifies a version 1.1.*. If the image previously
used an older version, specifying the new one causes a cache bust of apt-get update and ensure the installation of the new version. Listing packages on
each line can also prevent mistakes in package duplication.
In addition, when you clean up the apt cache by removing /var/lib/apt/lists
reduces the image size, since the apt cache is not stored in a layer. Since the
RUN statement starts with apt-get update, the package cache is always
refreshed prior to apt-get install.
Note: The official Debian and Ubuntu images automatically run
apt-get clean, so explicit invocation is not required.
Some RUN commands depend on the ability to pipe the output of one command into another, using the pipe character (|), as in the following example:
RUN wget -O - https://some.site | wc -l > /numberDocker executes these commands using the /bin/sh -c interpreter, which
only evaluates the exit code of the last operation in the pipe to determine
success. In the example above this build step succeeds and produces a new
image so long as the wc -l command succeeds, even if the wget command
fails.
If you want the command to fail due to an error at any stage in the pipe,
prepend set -o pipefail && to ensure that an unexpected error prevents
the build from inadvertently succeeding. For example:
RUN set -o pipefail && wget -O - https://some.site | wc -l > /numberNote: Not all shells support the
-o pipefailoption. In such cases (such as thedashshell, which is the default shell on Debian-based images), consider using the exec form ofRUNto explicitly choose a shell that does support thepipefailoption. For example:
RUN ["/bin/bash", "-c", "set -o pipefail && wget -O - https://some.site | wc -l > /number"]Dockerfile reference for the CMD instruction
The CMD instruction should be used to run the software contained by your
image, along with any arguments. CMD should almost always be used in the
form of CMD [“executable”, “param1”, “param2”…]. Thus, if the image is for a
service, such as Apache and Rails, you would run something like
CMD ["apache2","-DFOREGROUND"]. Indeed, this form of the instruction is
recommended for any service-based image.
In most other cases, CMD should be given an interactive shell, such as bash, python
and perl. For example, CMD ["perl", "-de0"], CMD ["python"], or
CMD [“php”, “-a”]. Using this form means that when you execute something like
docker run -it python, you’ll get dropped into a usable shell, ready to go.
CMD should rarely be used in the manner of CMD [“param”, “param”] in
conjunction with ENTRYPOINT, unless
you and your expected users are already quite familiar with how ENTRYPOINT
works.
Dockerfile reference for the EXPOSE instruction
The EXPOSE instruction indicates the ports on which a container listens
for connections. Consequently, you should use the common, traditional port for
your application. For example, an image containing the Apache web server would
use EXPOSE 80, while an image containing MongoDB would use EXPOSE 27017 and
so on.
For external access, your users can execute docker run with a flag indicating
how to map the specified port to the port of their choice.
For container linking, Docker provides environment variables for the path from
the recipient container back to the source (ie, MYSQL_PORT_3306_TCP).
Dockerfile reference for the ENV instruction
To make new software easier to run, you can use ENV to update the
PATH environment variable for the software your container installs. For
example, ENV PATH /usr/local/nginx/bin:$PATH ensures that CMD [“nginx”]
just works.
The ENV instruction is also useful for providing required environment
variables specific to services you wish to containerize, such as Postgres’s
PGDATA.
Lastly, ENV can also be used to set commonly used version numbers so that
version bumps are easier to maintain, as seen in the following example:
ENV PG_MAJOR 9.3
ENV PG_VERSION 9.3.4
RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && …
ENV PATH /usr/local/postgres-$PG_MAJOR/bin:$PATH
Similar to having constant variables in a program (as opposed to hard-coding
values), this approach lets you change a single ENV instruction to
auto-magically bump the version of the software in your container.
Each ENV line creates a new intermediate layer, just like RUN commands. This
means that even if you unset the environment variable in a future layer, it
still persists in this layer and its value can be dumped. You can test this by
creating a Dockerfile like the following, and then building it.
FROM alpine
ENV ADMIN_USER="mark"
RUN echo $ADMIN_USER > ./mark
RUN unset ADMIN_USER
CMD sh$ docker run --rm -it test sh echo $ADMIN_USER
markTo prevent this, and really unset the environment variable, use a RUN command
with shell commands, to set, use, and unset the variable all in a single layer.
You can separate your commands with ; or &&. If you use the second method,
and one of the commands fails, the docker build also fails. This is usually a
good idea. Using \ as a line continuation character for Linux Dockerfiles
improves readability. You could also put all of the commands into a shell script
and have the RUN command just run that shell script.
FROM alpine
RUN export ADMIN_USER="mark" \
&& echo $ADMIN_USER > ./mark \
&& unset ADMIN_USER
CMD sh$ docker run --rm -it test sh echo $ADMIN_USER
Although ADD and COPY are functionally similar, generally speaking, COPY
is preferred. That’s because it’s more transparent than ADD. COPY only
supports the basic copying of local files into the container, while ADD has
some features (like local-only tar extraction and remote URL support) that are
not immediately obvious. Consequently, the best use for ADD is local tar file
auto-extraction into the image, as in ADD rootfs.tar.xz /.
If you have multiple Dockerfile steps that use different files from your
context, COPY them individually, rather than all at once. This ensures that
each step's build cache is only invalidated (forcing the step to be re-run) if the
specifically required files change.
For example:
COPY requirements.txt /tmp/
RUN pip install --requirement /tmp/requirements.txt
COPY . /tmp/
Results in fewer cache invalidations for the RUN step, than if you put the
COPY . /tmp/ before it.
Because image size matters, using ADD to fetch packages from remote URLs is
strongly discouraged; you should use curl or wget instead. That way you can
delete the files you no longer need after they've been extracted and you don't
have to add another layer in your image. For example, you should avoid doing
things like:
ADD http://example.com/big.tar.xz /usr/src/things/
RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
RUN make -C /usr/src/things all
And instead, do something like:
RUN mkdir -p /usr/src/things \
&& curl -SL http://example.com/big.tar.xz \
| tar -xJC /usr/src/things \
&& make -C /usr/src/things all
For other items (files, directories) that do not require ADD’s tar
auto-extraction capability, you should always use COPY.
Dockerfile reference for the ENTRYPOINT instruction
The best use for ENTRYPOINT is to set the image's main command, allowing that
image to be run as though it was that command (and then use CMD as the
default flags).
Let's start with an example of an image for the command line tool s3cmd:
ENTRYPOINT ["s3cmd"]
CMD ["--help"]
Now the image can be run like this to show the command's help:
$ docker run s3cmd
Or using the right parameters to execute a command:
$ docker run s3cmd ls s3://mybucket
This is useful because the image name can double as a reference to the binary as shown in the command above.
The ENTRYPOINT instruction can also be used in combination with a helper
script, allowing it to function in a similar way to the command above, even
when starting the tool may require more than one step.
For example, the Postgres Official Image
uses the following script as its ENTRYPOINT:
#!/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 "$@"
fi
exec "$@"Note: This script uses the
execBash command so that the final running application becomes the container's PID 1. This allows the application to receive any Unix signals sent to the container. See theENTRYPOINThelp for more details.
The helper script is copied into the container and run via ENTRYPOINT on
container start:
COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
CMD ["postgres"]
This script allows the user to interact with Postgres in several ways.
It can simply start Postgres:
$ docker run postgres
Or, it can be used to run Postgres and pass parameters to the server:
$ docker run postgres postgres --help
Lastly, it could also be used to start a totally different tool, such as Bash:
$ docker run --rm -it postgres bash
Dockerfile reference for the VOLUME instruction
The VOLUME instruction should be used to expose any database storage area,
configuration storage, or files/folders created by your docker container. You
are strongly encouraged to use VOLUME for any mutable and/or user-serviceable
parts of your image.
Dockerfile reference for the USER instruction
If a service can run without privileges, use USER to change to a non-root
user. Start by creating the user and group in the Dockerfile with something
like RUN groupadd -r postgres && useradd --no-log-init -r -g postgres postgres.
Note: Users and groups in an image get a non-deterministic UID/GID in that the “next” UID/GID gets assigned regardless of image rebuilds. So, if it’s critical, you should assign an explicit UID/GID.
Note: Due to an unresolved bug in the Go archive/tar package's handling of sparse files, attempting to create a user with a sufficiently large UID inside a Docker container can lead to disk exhaustion as
/var/log/faillogin the container layer is filled with NUL (\0) characters. Passing the--no-log-initflag to useradd works around this issue. The Debian/Ubuntuadduserwrapper does not support the--no-log-initflag and should be avoided.
Avoid installing or using sudo since it has unpredictable TTY and
signal-forwarding behavior that can cause problems. If
you absolutely need functionality similar to sudo, such as initializing the
daemon as root but running it as non-root), consider using
“gosu”.
Lastly, to reduce layers and complexity, avoid switching USER back
and forth frequently.
Dockerfile reference for the WORKDIR instruction
For clarity and reliability, you should always use absolute paths for your
WORKDIR. Also, you should use WORKDIR instead of proliferating
instructions like RUN cd … && do-something, which are hard to read,
troubleshoot, and maintain.
Dockerfile reference for the ONBUILD instruction
An ONBUILD command executes after the current Dockerfile build completes.
ONBUILD executes in any child image derived FROM the current image. Think
of the ONBUILD command as an instruction the parent Dockerfile gives
to the child Dockerfile.
A Docker build executes ONBUILD commands before any command in a child
Dockerfile.
ONBUILD is useful for images that are going to be built FROM a given
image. For example, you would use ONBUILD for a language stack image that
builds arbitrary user software written in that language within the
Dockerfile, as you can see in Ruby’s ONBUILD variants.
Images built from ONBUILD should get a separate tag, for example:
ruby:1.9-onbuild or ruby:2.0-onbuild.
Be careful when putting ADD or COPY in ONBUILD. The “onbuild” image
fails catastrophically if the new build's context is missing the resource being
added. Adding a separate tag, as recommended above, helps mitigate this by
allowing the Dockerfile author to make a choice.
These Official Repositories have exemplary Dockerfiles: