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Guide to production docker images

25 Oct 2022

What makes a good production image

I would like to cover initial basics agnostic to Linux distrobution or recommended base images. The intent here is to start by defining what makes a good Docker image for an application.

  • The Docker image should only have dependencies necessary for running the application. This provides a few benefits.
    • Fewer extra software means improved security.
    • A smaller Docker image which means your app or API starts faster in cloud provisioning services because it is fast to download.
  • The app should log to stdout and stderr. Not write logs to disk. This is important because Docker can handle log management but not if it is written to disk.
  • The app should launch in the foreground and manage its own child processes from there. Docker does a good job at process management. Let Docker manage passing signals to your application.
  • Even if launched in the foreground, you’ll still want the docker image to have a process dedicated to PID 1 functions. If you’re not familiar with this issue I recommend the original phusion blog post calling out the problem. My preference is to bake in Yelp dumb-init for a couple of reasons. dumb-init is small and I don’t have to worry about the orchestration service supporting --init flags for Docker.
    • If not addressing this issue while in production, you’ll find that occasionally your service will auto-scale. The auto-scaling could be a symptom due to being constrained with resources. It won’t be immediately clear without good monitoring and easy to mistake for traffic balancing.
  • When your application exits, provide a meaningful exit code. Zero for success and non-zero for failure. Depending on the complexity of your application it might assist troubleshooting if you provide documentation around failure exit codes. The documentation could even include tips on what went wrong and how to resolve it such as a run book for a support team.
  • Your application should run as a normal user and not root user inside of the container. This improves security of both your app and the host in which it resides.
  • Your application should start the container with default Linux capabilities provided by Docker. If possible, try not to add any extra capabilities which would reduce the security if your application runtime.
  • If your app requires outbound network connectivity involving TLS, then it should include standard TLS certificate authorities for whatever cloud or platform you’re working from.
  • If your app requires timezone information then you should include Linux timezone files.
  • Even if you’re using a minimal image, you’ll want to follow the Linux filesystem hierarchy standard and provide all of the above recommendations even if building from scratch. I will provide an example at the end of this article.
  • If you add set -ex at the beginning of your RUN commands you get better debug output from /bin/sh while Docker is building the image. This helps narrow down issues to the exact command that fails when a Docker build fails. It also forces a multi-statement RUN command to exit with an error without the need to use && between commands.

There are other good practices in general for applications such as integrating application performance monitoring (APM), unit testing with code coverage, documentation, and data flow diagrams. However, this article is mostly highlighting Docker so I don’t plan to dive into these other topics here. They are still good to build in as observability will add to service quality.

An example application

  • Flask is a framework based on the current/old standard for Python web frameworks: WSGI.
  • FastAPI is based on Starlette, which uses the newer standard for asynchronous web frameworks: ASGI.

For the purposes of this write-up I developed a simple flask application (a REST API with only one endpoint from the flask REST API tutorial). If you’re starting an API from scratch you might want to consider FastAPI, instead. However, for the purposes of this writeup the application or framework doesn’t matter. The real star is showing examples of production ready Docker images.

You can go view the example at docker-production-ready-flask. It follows recommendations from Flask documentation on how to deploy flask to production paired with Docker best practices. The project README goes into more detail.

Diving into “from scratch”

Ivan Velichko has a great writeup on, why Google distroless instead of “from scratch”? The short version of the article is the following.

FROM scratch images are bad (e.g. minimal go apps from scratch) because:

  • They can’t run as non-root.
  • Timezone information is missing so you will have Go bugs encountered at runtime.
  • Standard directories (e.g. /var/tmp and /tmp) are missing which cause issues with native Go calls that rely on temporary files.

Let’s inspect and verify. For the purposes of inspection I will pull in a statically compiled version of bash.

Here’s our minimal Dockerfile for inspection.

FROM alpine
ADD https://github.com/robxu9/bash-static/releases/download/5.1.016-1.2.3/bash-linux-x86_64 /bash
RUN chmod 755 /bash
FROM scratch
COPY --from=0 /bash /

Now to build and look around.

docker build -t minimal .

Launch the container.

docker run -it --rm minimal /bash

Look around. There’s no standard GNU utilities so we’ll be limited to shell built-in functions.

bash-5.1# echo *
bash dev etc proc sys

bash-5.1# echo dev/* 
dev/console dev/core dev/fd dev/full dev/mqueue dev/null dev/ptmx dev/pts
dev/random dev/shm dev/stderr dev/stdin dev/stdout dev/tty dev/urandom dev/zero

bash-5.1# echo etc/*
etc/hostname etc/hosts etc/mtab etc/resolv.conf

bash-5.1# echo $$
1

Some observations:

  • It is a minimal container.
  • bash is our shell and proc/sys are kernel filesystems so no need to inspect them since they are kernel API mounts.
  • Because the bash prompt has # it means we are running as root user.
  • We don’t need to worry about standard devices or filesystems like /dev/shm, /dev/zero, etc.
  • We are missing temporary directories such as /tmp and /var/tmp.
  • We’re running as PID 1 (verified with echo $$). Bash can’t handle PID 1 signals. Any app you put into a from scratch image will also unlikely handle signals unless the app is explicitly designed for it.
  • /etc has standard networking mounts provided by Docker but we don’t have standard user files like /etc/passwd and /etc/group.

Creating our most minimal Docker container

Accounting for best practices (except for TLS certificates and tzinfo) let’s create an example Dockerfile.

ARG base=alpine
FROM ${base}

RUN set -ex; \
  # Prerequisites
  apk add --no-cache build-base; \
  # Directory structure and permissions
  mkdir -p base/bin base/tmp base/var/tmp base/etc base/home/nonroot base/sbin base/root; \
  chmod 700 /root; \
  chown root:root /root; \
  chmod 1777 base/tmp base/var/tmp; \
  chown 65532:65532 base/home/nonroot; \
  chmod 750 base/home/nonroot; \
  # UID and GID
  echo 'root:x:0:' > /base/etc/group; \
  echo 'nonroot:x:65532:' >> /base/etc/group; \
  echo 'root:x:0:0:root:/root:/sbin/nologin' > /base/etc/passwd; \
  echo 'nonroot:x:65532:65532:nonroot:/home/nonroot:/sbin/nologin' >> /base/etc/passwd; \
  # init binary for PID 1
  wget -O base/bin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.5/dumb-init_1.2.5_"`uname -m`"; \
  chmod 755 base/bin/dumb-init; \
  # nologin binary
  echo 'int main() { return 1; }' > nologin.c; \
  gcc -Os -no-pie -static -std=gnu99 -s -Wall -Werror -o base/sbin/nologin nologin.c; \
  echo "Minimal Container version $VERSION" > /etc/issue


# Add our example program (bash)
# Note: you don't need this for your own application.  In this case static bash
#   is the example application running in user context within a minimal image
# Comment out these lines and update CMD for your own app.
RUN set -ex; \
  wget -O base/bin/bash https://github.com/robxu9/bash-static/releases/download/5.1.016-1.2.3/bash-linux-"`uname -m`"; \
  chmod 755 base/bin/bash

FROM scratch
COPY --from=0 /base/ /
ENTRYPOINT ["/bin/dumb-init", "--"]
USER nonroot
ENV HOME=/home/nonroot USER=nonroot
WORKDIR /home/nonroot
CMD ["/bin/bash"]

Build it.

docker build -t minimal .

Let’s look around!

docker run -it --rm minimal
bash-5.1$

bash-5.1$ echo $$
7

bash-5.1$ /proc/1/exe --version
dumb-init v1.2.5

bash-5.1$ pwd
/home/nonroot

bash-5.1$ while IFS=$'\0' read -r -d $'\0' line; do echo "$line"; done < /proc/self/environ
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=7cdf06c28dd1
TERM=xterm
HOME=/home/nonroot
USER=nonroot

bash-5.1$ echo /sbin/* /bin/* /var/* /t*
/sbin/nologin /bin/bash /bin/dumb-init /var/tmp /tmp

bash-5.1$ cd /; echo *
bin dev etc home proc root sbin sys tmp var

bash-5.1$ echo hello > /tmp/file

bash-5.1$ echo /tmp/*
/tmp/file

Observations

  • We’re running as a normal user because the bash prompt has a dollar sign $ (not root).
  • Our shell is no longer PID 1.
  • We used the /proc kernel file system to verify that PID 1 is the dumb-init process. If you’re curious about /proc you can read about it within the kernel doc proc.txt.
  • We are in the nonroot home directory and our environment reflects the user environment.
  • We have our binaries in a standard paths. Following the Linux filesystem hierarchy standard.
  • /sbin/nologin will return non-zero if a user login is attempted.
  • As a user, we can write to the /tmp filesystem verifying its permissions are set correctly along with the sticky bit.

ARM support

This minimal Docker image is meant to be cross platform for both AMD64 and ARM64.

docker buildx build --platform linux/arm64 --build-arg base=arm64v8/alpine -t minimal-arm .

Or if you’re on a computer which is already native ARM you can run the original docker command and it should build just fine.

docker build -t minimal .

Full Docker example with TLS CA and tzinfo

I work a lot with Amazon web services. In my case, it makes sense to copy certificates and timezone information from amazonlinux:2. However, if you’re in another cloud provider or data center; then use whatever base image of your choice. Copying these paths are pretty standard for nearly any Linux distribution.

ARG base=alpine
FROM ${base}

RUN set -ex; \
  # Prerequisites
  apk add --no-cache build-base; \
  # Directory structure and permissions
  mkdir -p base/bin base/tmp base/var/tmp base/etc base/home/nonroot base/sbin base/root; \
  chmod 700 /root; \
  chown root:root /root; \
  chmod 1777 base/tmp base/var/tmp; \
  chown 65532:65532 base/home/nonroot; \
  chmod 750 base/home/nonroot; \
  # UID and GID
  echo 'root:x:0:' > /base/etc/group; \
  echo 'nonroot:x:65532:' >> /base/etc/group; \
  echo 'root:x:0:0:root:/root:/sbin/nologin' > /base/etc/passwd; \
  echo 'nonroot:x:65532:65532:nonroot:/home/nonroot:/sbin/nologin' >> /base/etc/passwd; \
  # init binary
  wget -O base/bin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.5/dumb-init_1.2.5_"`uname -m`"; \
  chmod 755 base/bin/dumb-init; \
  # nologin binary
  echo 'int main() { return 1; }' > nologin.c; \
  gcc -Os -no-pie -static -std=gnu99 -s -Wall -Werror -o base/sbin/nologin nologin.c; \
  echo "Minimal Container version $VERSION" > /etc/issue


# Add our example program (bash)
# Note: you don't need this for your own application.  In this case static bash
#   is the example application running in user context within a minimal image
# Comment out these lines and update CMD for your own app.
RUN set -ex; \
  wget -O base/bin/bash https://github.com/robxu9/bash-static/releases/download/5.1.016-1.2.3/bash-linux-"`uname -m`"; \
  chmod 755 base/bin/bash

# Pull TLS certifactes and timezone info from amazon
FROM amazonlinux:2
RUN set -ex; \
  mkdir -p base/etc base/usr/share; \
  cp -r /etc/ssl /etc/pki base/etc/; \
  cp -r /usr/share/zoneinfo base/usr/share/


FROM scratch
COPY --from=0 /base/ /
COPY --from=1 /base/ /
ENTRYPOINT ["/bin/dumb-init", "--"]
USER nonroot
ENV HOME=/home/nonroot USER=nonroot
WORKDIR /home/nonroot
CMD ["/bin/bash"]

Summary

If you’re building statically compiled binaries then you can rely on all of the recommended best practices for Linux and Docker to have both a safer and smaller Docker image than one provided to you by a Linux distribution.

Here’s a small summary of the image sizes. All sized exclude bash assuming you would remove it to replace it with a statically compiled application.

  • 76.4kB minimal example. If your app doesn’t need CA certificates or timezone information then you get all of the security and stability goodies at a very small storage price.
  • 3.53MB when including TLS CA certificates and timezone information.

This article was last updated Oct 25, 2022.

Posted in Engineering with tags: Programming, Linux, Tips


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