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Dockerfile Code Style

Editor Settings

  • Your editor must be able to recognize the .editorconfig file in project.

Follow the Best practices for writing Dockerfiles

NO "upgrade" in NON-OS-Base-Images

While you MAY "update" the file-lists of the package manager of the OS in the Base Image you have chosen, you MUST NOT alter the contents of a Base Image by "upgrading" it in a Dockerfile build.

This is OK

FROM ubuntu: 14.04.1
RUN apt-get update \
	&& apt-get install -y ...
# ...

This is BAD

FROM ubuntu: 14.04.1
RUN apt-get update \
	&& apt-get upgrade -y
	&& apt-get install -y ...
# ...

The apt-get upgrade (or the command that does the trick in "your" Base Image) might change installed software versions or even install or remove packages based on new dependencies. The resulting Docker Image does NOT match the Base Image anymore.

If you NEED such statements because the docker build fails, ASK for a new upgraded Base Image

Not leave unnecessary files and tools in layer

  • MUST NOT leave configuration management tools (puppet, ansible, chef, ...) INSIDE a image layer.

Use only well maintained images FROM a trusted registry

We trust "Docker" and "ourselves". As a consequence, you can pull images from, or

NO secrets in Dockerfile (or its repository)

A VERY bad example of a Dockerfile would be

FROM ubuntu:14.04.1
ENV API_KEY=4711101 \
	USR=admin \
	PWD=the_admin_password \
	CREDITCARD=5555 3333 2222
ADD the_same_secrets.txt /etc
# ...

You MUST keep secrets out of the services Dockerfile AND its repository.

Dockerfile repositories with all their accompanying files tend to be cloned, shared, etc. IF you forget about the secrets inside, anybody who gains access to the repository would immediately KNOW your precious secrets.

You can solve this dilemma by

  • provide secrets as environment variables when you start the container with docker run -e API_KEY=4711 ...
  • create strictly separate repositories for secret/configuration containers that are NEVER at risk being published outside
  • create configuration only containers "on the fly" during provisioning
  • let the containers entrypoint or command retrieve secrets at startup.

VERIFY downloaded artifacts

When using the Dockerfile with the docker build command, files from the local filesystem (the build context) can be used or being retrieved from services in the intra- or internet.

You MUST verify externally downloaded files.

VERIFY downloaded artifacts (I)

You SHOULD use one of:

  • a signature for the external file itself
  • a signed file with the HASH for the external file and the signing key from a public keyserver.

Using a keyserver with HTTP support (port 80) might help with problematic firewalls.

 # ...
 # ...
RUN curl -sL > \
    && curl -sL > app.sha \
    && curl -sL > app.sig \
    && gpg --keyserver --recv-keys ${HASHICORP_KEYIDS} \
    && gpg --verify app.sig app.sha \
    && grep linux_amd64 app.sha | sha256sum -sc \
    && echo "We only get here if sha256sum succeeded ..."

VERIFY downloaded artifacts (II)

If there is no better option, you SHOULD use a well known HASH from a trusted source (e.g. placed directly in the Dockerfile)

 # ...
ENV VAULT_SHA256=f81accce15313881b8d53b039daf090398b2204b1154f821a863438ca2e5d570
 # ...
RUN curl -sL > /tmp/my_file \
    && echo "${VAULT_SHA256}  /tmp/my_file" | sha256sum -sc  \
    && echo "We only get here if sha256sum succeeded ..."

No "root" processes in containers (please)

A process inside a container SHOULD NOT run as root (with uid “0”) to reduce possible runtime risks - especially in respect to (root-owned ?) volumes from the host. Even a Docker Engine > 1.10 (with new security features enabled) is a misconfiguration risk to be avoided.

If possible, set the effective runtime-user in the dockerfile.

host$ cat dockerfile
FROM ubuntu

# Installing as almighty "root"
RUN ...
COPY ...

# now switching to harmless "guest" runtime user
USER guest

CMD ["/"]

Some programs - like Apache or Nginx - have to be STARTED as root, but provide configuration settings to switch to a different, much less risky runtime user.

Only EXPOSE necessary ports

In a Dockerfile, you MUST only EXPOSE ports that are required from outside of the container. If a program provides its API via port 80 AND 443, but you don't really need port 80, this should be reflected in the Dockerfile:

# Disabled


Write Log/Error to Stdout/Stderr

The container MUST use this functionality IF POSSIBLE to support a standardized log management approach.

Stdout SHOULD contain regular information about the process doings, while stderr SHOULD be restricted to messages about technical errors or warnings that an operator or admin has to take care of.

Such messages SHOULD be machine readable (e.g. json formatted) to avoid extensive parsing steps in later processing.

Of course, the application MAY uses other (additional) means of log forwarding but is immediately on its own for maintenance, configuration etc.

Use Exec form rather than Shell

You MUST write CMD and ENTRYPOINT statements using the Exec-form. Using the shell form would effectively trigger a call to /bin/sh -c with the command and parameters you specified.

As a result, when you use docker stop or docker kill the POSIX signal will only be sent to the container process running as PID 1, and if that process is /bin/sh rather than the underlying process you started and /bin/sh doesn't forward signals to any child processes you won't be able to gracefully stop the process.

Use tini as PID 1


One Process/Service per Container

A Dockerfile MUST only install packages for ONE logical service.

A logical service SHOULD be only a SINGLE process like one web- or database server.

FROM ubuntu:14.04.1
CMD [ "/opt/myservice/myprogram" ]

The technical reason is the STREAMS (Stdin/Stdout/Stderr) and SIGNAL handling between the Docker Engine and the process(es) running in a Docker Container. When not constructed/handled carefully, something similar to "unix process zombies" might raise their ugly heads ...

This asks for installing and starting applications the "right way"

There are solutions for cleanly handling multiple processes inside a container like Using Supervisor with Docker,

Inspired & Thanks

This documents are modified from Haufe-Lexware/docker-style-guide which under Apache-2.0 license. Thanks for their great .