Dockerfile Best Practices

Writing production-ready Dockerfiles is not as simple as you could think about it.

This repository contains some best-practices for writing Dockerfiles. Most of them are coming from my experience while working with Docker & Kubernetes. This is all guidance, not a mandate - there may sometimes be reasons to not do what is described here, but if you don't know then this is probably what you should be doing.

Best practices included in the Dockerfile

The following are included in the Dockerfile in this repository:

Use official Docker images whenever possible
Alpine is not always the best choice
Limit image layers amount
Run as a non-root user
Do not use a UID below 10,000
Use a static UID and GID
The latest is an evil, choose specific image tag
Only store arguments in CMD
Always use COPY instead of ADD (there is only one exception)
Always combine RUN apt-get update with apt-get install in the same run statement

Use official Docker images whenever possible

Using official Docker images dedicated for your technology should be the first choice. Why? Because those images are optimized and tested by MILLIONS of users. Creating your image from scratch is a good idea when official base image contains vulnerabilities or you cannot find a base image dedicated for your technology.

Instead of installing SDK manually:

FROM ubuntu
RUN wget -qO- https://packages.microsoft.com/keys/microsoft.asc \
| gpg --dearmor > microsoft.asc.gpg \
&& sudo mv microsoft.asc.gpg /etc/apt/trusted.gpg.d/ \
&& wget -q https://packages.microsoft.com/config/ubuntu/18.04/prod.list \
&& sudo mv prod.list /etc/apt/sources.list.d/microsoft-prod.list \
&& sudo chown root:root /etc/apt/trusted.gpg.d/microsoft.asc.gpg \
&& sudo chown root:root /etc/apt/sources.list.d/microsoft-prod.list
RUN sudo apt-get install dotnet-sdk-3.1

Use official Docker Image for dotnet

FROM mcr.microsoft.com/dotnet/core/sdk:3.1-buster

Alpine is not always the best choice

Even though Alpine is lightweight, there are some known issues with performance for some technologies (https://pythonspeed.com/articles/alpine-docker-python/).

Second thing about Alpine-based images - security. Most of vulnerabilities scanners do not find any vulnerabilities in Alpine-based images. If scanner didn't find any vulnerability, does it mean it's 100% secure? Of course not.

Before making a decision, evaluate what are benefits of alpine.

Limit image layers amount

Each RUN instruction in your Dockerfile will end up creating an additional layer in your final image. The best practice is to limit the amount of layers to keep the image lightweight.

Instead of:

RUN curl -SL "https://nodejs.org/dist/v${NODE_VERSION}/node-v${NODE_VERSION}-linux-x64.tar.gz" --output nodejs.tar.gz
RUN echo "$NODE_DOWNLOAD_SHA nodejs.tar.gz" | sha256sum -c -
RUN tar -xzf "nodejs.tar.gz" -C /usr/local --strip-components=1
RUN rm nodejs.tar.gz
RUN ln -s /usr/local/bin/node /usr/local/bin/nodejs

Use this:

RUN curl -SL "https://nodejs.org/dist/v${NODE_VERSION}/node-v${NODE_VERSION}-linux-x64.tar.gz" --output nodejs.tar.gz \
&& echo "$NODE_DOWNLOAD_SHA nodejs.tar.gz" | sha256sum -c - \
&& tar -xzf "nodejs.tar.gz" -C /usr/local --strip-components=1 \
&& rm nodejs.tar.gz \
&& ln -s /usr/local/bin/node /usr/local/bin/nodejs

Run as a non-root user

Running containers as a non-root user substantially decreases the risk that container -> host privilege escalation could occur. This is an added security benefit (Docker docs).

For Debian-based images, removing root from container can be done like this:

RUN groupadd -g 10001 dotnet && \
   useradd -u 10000 -g dotnet dotnet \
   && chown -R dotnet:dotnet /app

USER dotnet:dotnet

NOTE: Sometimes when you remove the root from container, you will need to adjust your application/service permissions.

For example, dotnet applications cannot run on port 80 without root privileges and you have to change the default port (in the example it's 5000).

ENV ASPNETCORE_URLS http://*:5000

Do not use a UID below 10,000

UIDs below 10,000 are a security risk on several systems, because if someone does manage to escalate privileges outside the Docker container their Docker container UID may overlap with a more privileged system user's UID granting them additional permissions. For best security, always run your processes as a UID above 10,000.

Use a static UID and GID

Eventually someone dealing with your container will need to manipulate file permissions for files owned by your container. If your container does not have a static UID/GID, then one must extract this information from the running container before they can assign correct file permissions on the host machine. It is best that you use a single static UID/GID for all of your containers that never changes. We suggest 10000:10001 such that chown 10000:10001 files/ always works for containers following these best practices.

The `latest` is an evil, choose specific image tag

Use a specific image version using major.minor, not major.minor.patch so as to ensure you are always:

Keeping your builds working (latest means your build can arbitrarily break in the future, whereas major.minor should mean this doesn't happen)
Getting the latest security updates included in new images you build.

Why you perhaps shouldn't pin with a SHA

SHA pinning gives you completely reliable and reproducible builds, but it also likely means you won't have any obvious way to pull in important security fixes from the base images you use. If you use major.minor tags, you get security fixes by accident when you build new versions of your image - at the cost of builds being less reproducible.

Consider using docker-lock: this tool keeps track of exactly which Docker image SHA you are using for builds, while having the actual image you use still be a major.minor version. This allows you to reproduce your builds as if you'd used SHA pinning, while getting important security updates when they are released as if you'd used major.minor versions.

Only store arguments in `CMD`

By having your ENTRYPOINT be your command name:

ENTRYPOINT ["/sbin/tini", "--", "myapp"]

And CMD be only arguments for your command:

CMD ["--foo", "5", "--bar=10"]

It allows people to ergonomically pass arguments to your binary without having to guess its name, e.g. they can write:

docker run IMAGE --some-argument

If CMD includes the binary name, then they must guess what your binary name is in order to pass arguments etc.

Always use COPY instead of ADD (there is only one exception)

The only one exception for using ADD is tar auto-extraction capability

ADD local-file.tar.xz /usr/share/files

Arbitrary URLs specified for ADD could result in MITM attacks, or sources of malicious data. In addition, ADD implicitly unpacks local archives which may not be expected and result in path traversal and Zip Slip vulnerabilities.

You should avoid doing like this:

ADD https://example-url.com/file.tar.xz /usr/share/files

To summarize, COPY should be used whenever possible.

Always combine RUN `apt-get update` with `apt-get install` in the same run statement

Using apt-get update alone in a RUN causes caching issues and subsequent apt-get install instructions fail. It’s related to caching mechanism that Docker use. While building the image, 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, the build can potentially get an outdated version of packages.

Below is an example of RUN instruction that demonstrates all the apt-get recommendations.

RUN apt-get update && apt-get install -y \
    curl \
    git \
    build-essential  \
    && rm -rf /var/lib/apt/lists/*

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