Migrating Dockerfiles To Chainguard

To convert a Dockerfile to use Chainguard images you should follow these steps:

1. Clarify instructions with the user
2. Convert the Dockerfile
   1. Modify FROM statements to use Chainguard images.
   2. Migrate any uses of other OS package managers to apk
   3. Adjust users and permissions where required.
3. Test the conversion
   1. Build the image
   2. Run the image
   3. Troubleshoot any failures

Clarify Instructions

If you're asked to convert a Dockerfile to Chainguard, before doing anything else, you should clarify the following with the user:

1. What is the Chainguard organization name? Each Chainguard customer has their own organization and it will be referred to generically as ORGANIZATION in these instructions.
2. Is FIPS required? Chainguard offers FIPS compliant images that have the suffix -fips. For instance: python-fips.
3. Should images be pulled directly from cgr.dev? Or, if images are hosted/proxied from another repository, what is that repository?
4. Should you test the conversion?
   1. Should you try and build the image to validate the conversion worked?
   2. Should you try and run the image to validate the conversion worked?

Store these answers under ~/.claude/chainguard-preferences.md and try and reuse this file across subsequent asks to migrate Dockerfiles.

Backup the Dockerfile

You should backup the old Dockerfile by prepending old. to it. For instance, Dockerfile -> old.Dockerfile.

Then you can modify the original file directly.

Converting FROM statements

You should replace images defined in FROM lines with the equivalent Chainguard image.

Always use the mapping-container-images-to-chainguard skill if it is available.

You should try to match to actively maintained tags when converting Dockerfiles.

You should prefer -dev tags as they have more utilities in them and they are less likely to cause issues. One known exception is chainguard-base which has no -dev variant. You should always use the latest tag for chainguard-base.

Digests

If the existing reference uses a digest reference (i.e python:3.12-slim-bookworm@sha256:a866731a6b71c4a194a845d86e06568725e430ed21821d0c52e4efb385cf6c6f) then you should also include a digest. Naturally, the digest will be different for the Chainguard image. You can figure out the digest in a few ways, depending on the available tooling:

# With crane (prefer this if it is available)
crane digest cgr.dev/ORGANIZATION/python:3.12-dev

# Or, with docker
docker pull cgr.dev/ORGANIZATION/python:3.12-dev
docker inspect --format='{{index .RepoDigests 0}}' cgr.dev/ORGANIZATION/python:3.12-dev

Chainguard Base

Chainguard have their own Linux distribution, so they don't have equivalent images for alpine, debian, ubuntu etc.

Where the Dockerfile is using one of these generic bases, you should swap it to use chainguard-base (or chainguard-base-fips for FIPS). You should always use the latest tag with chainguard-base. There is no latest-dev tag for chainguard-base, so don't use that.

Multi Stage Builds

Dev Variants

Where there are multiple FROM lines in a Dockerfile, you should try to use a non-dev variant (i.e cgr.dev/ORGANIZATION/python:3.12) for the final stage unless there are any RUN lines that follow it (in which case you need a shell, and therefore a -dev image).

COPY Permissions

A common issue with multi stage builds is that files are created in the first stage by the root user, which are then not accessible to the 65532 user in the final stage. You can mitigate this by ensuring files copied into the final stage belong to the 65532 user:

COPY --from=build --chown=65532:65532 /app .

Adding Packages with apk

Unlike other Linux distributions that may use apt, yum or dnf to install packages, Chainguard images use apk to manage packages.

This means that you should convert instances of apt, yum and dnf to apk in RUN lines.

For instance, for apt, lines like this:

RUN apt-get update \
    && apt-get install -y software-properties-common=0.99.22.9 \
    && add-apt-repository ppa:libreoffice/libreoffice-still \
    && apt-get install -y libreoffice \
    && apt-get clean

Should become:

RUN apk --no-cache add libreoffice

And lines like this:

RUN yum update -y && \
    yum -y install httpd php php-cli php-common && \
    yum clean all && \
    rm -rf /var/cache/yum/*

RUN dnf update -y && \
    dnf -y install httpd php php-cli php-common && \
    dnf clean all

RUN microdnf update -y && \
    microdnf -y install httpd php php-cli php-common && \
    microdnf clean all

Should become:

RUN apk add --no-cache httpd php php-cli php-common

Don't just remove apt lines. Replace them with apk equivalents.

Root Permissions

Chainguard images typically run as a non root user with the UID 65532, rather than root. You will need to switch to root before running apk add and then revert back to 65532 after.

USER root
RUN apk add --no-cache httpd php php-cli php-common
USER 65532

Users

As mentioned above, Chainguard images typically run as a non root user with the UID 65532, rather than root. Commonly, this user is called nonroot, but not always. For instance, in the node image the user is called node.

To be safe, always prefer the UID when issuing USER statements or changing the permissions of files.

You should switch to the root user with USER root before running privileged commands (like npm) and then use USER 65532 after to return to the non root user.

You can figure out which user the image is configured to use by querying the image configuration. How you do that depends on the tools you have available:

# With crane.
crane config cgr.dev/chainguard/python | jq -r '.config.User'

# With docker.
docker inspect --format='{{ .Config.User }}' cgr.dev/chainguard/python

You can also find this information in the Specifications tab of the Chainguard Image Directory:

https://images.chainguard.dev/directory/image/python/specifications

Entrypoints

Unlike upstream images, the entrypoint for Chainguard base images is typically the runtime interpreter for the given language (i.e java, python, php) rather than a shell like sh or bash.

This means that a CMD like CMD ["python", "app.py"] will cause an error in a Chainguard images because it translates to running python python app.py.

We should prefer to use explicit entrypoints like:

ENTRYPOINT ["python", "app.py"]

PHP

It's common in Dockerfiles that build PHP applications to install composer in this way:

COPY --from=composer:latest /usr/bin/composer /usr/local/bin/composer

This is unnecessary when using the Chainguard php image because composer is already included in the -dev tags. Therefore, you can remove any lines that install composer.

Troubleshooting

Missing Shell

If you get errors complaining about a missing sh or bash, then ensure you are using a -dev image, which provides a shell.

Permission Denied

If you get errors complaining about permissions, then it's probably to do with the non root user. Switch to USER root temporarily and then switch back to USER 65532 once you've completed the privileged operation.

Check the Overview page

The Overview page for the Chainguard image typically includes information that is helpful when performing a migration.

https://images.chainguard.dev/directory/image/<image-name>/overview

If you run into issues, it is worth checking this page for guidance.