Migrating Dockerfiles to Chainguard Images

Based on the Wolfi Linux undistro, Chainguard Images have special features designed for increased security and provenance attestation. Depending on your current base image and custom commands, you may need to make some adjustments when migrating your current Dockerfile workloads to use Chainguard Images.

A general migration process would involve the following steps:

1. Identify the base image you need. Check out the Chainguard Images Directory to identify the image that is the closest match to what you currently use. You may also use wolfi-base as a flexible starting point for your experimentation.
2. Try the -dev variant of the image first. Chainguard Images typically have a distroless variant, which is very minimal and doesn’t include apk, and a dev variant that contains tooling necessary to build applications and install new packages. Start with the dev variant or the wolfi-base image to have more room for customization.
3. Identify packages you need to install. Depending on your current base image, you may need to include additional packages to meet dependencies. Refer to the Searching for Packages section for more details on how to find packages.
4. Migrate to a distroless image. Evaluate the option of using a Docker multi-stage build to create a final distroless image containing only what you need. Check the Getting Started with Distroless images for more details of how to work with distroless images. Although not required, this process should give you a smaller image with additional safeguards.

There are some differences in Wolfi’s busybox and coreutils packages when compared to their counterparts in distros such as Debian or even Alpine. Some binaries and scripts are not included by default, which contributes to a smaller package size. This was done in order to keep images to a minimum, but be aware that some commands might still be available through separate packages.

The next sections of this page contain distro-specific information that should help you streamline the migration process from your current base images to Chainguard images.

Migrating from Debian and Ubuntu Dockerfiles

Chainguard Images are based on the Alpine apk ecosystem, which differs from Debian-based apt in several aspects. Some of these features contribute in making packages smaller and more accountable, resulting in smaller images with traceable provenance information based on cryptographic signatures. The page Why apk from the official Wolfi documentation explains in more detail why we use apk.

If you are coming from a Debian-based Dockerfile, you’ll need to adapt some of your commands to be compatible with the apk ecosystem:

Our Debian Compatibility page has a table listing common tools and their corresponding package(s) in both Wolfi and Debian distributions. For Ubuntu-based Dockerfiles, check our Ubuntu Compatibility page.

Migrating from Red Hat UBI Dockerfiles

If you are coming from a Red Hat UBI (Universal Base Image) Dockerfile, you’ll also need to adapt some of your commands to be compatible with the apk ecosystem. Wolfi uses BusyBox utilities, which offer a smaller footprint compared to GNU coreutils in Red Hat images. Our Red Hat Compatibility page has a table listing common tools and their corresponding package(s) in both Wolfi and Red Hat distributions.

If you are coming from a Red Hat UBI based Dockerfile, you’ll need to adapt some of your commands to be compatible with the apk ecosystem:

Migrating from Alpine Dockerfiles

If your Dockerfile is based on Alpine, the process for migrating to Chainguard Images should be more straightforward, since you’re already using apk commands. Wolfi packages typically match what is available in Alpine, with some exceptions. For instance, the Wolfi busybox package is slimmer and doesn’t include all tools available in Alpine’s busybox. Check the Alpine Compatibility page for a list of common tools and their corresponding packages in Wolfi and Alpine.

Searching for Packages

Packages from Debian and other base distributions might have a different name in Wolfi. To search for packages, log into an ephemeral container based on cgr.dev/chainguard/wolfi-base:

docker run -it --rm --entrypoint /bin/sh cgr.dev/chainguard/wolfi-base

Then, run apk update to update the local apk cache with latest Wolfi packages:

apk update

You’ll get output similar to this:

fetch https://packages.wolfi.dev/os/x86_64/APKINDEX.tar.gz
[https://packages.wolfi.dev/os]
OK: 46985 distinct packages available

Now you can use apk search to look for packages. The following example searches for PHP 8.2 XML extensions:

apk search php*8.2*xml*

You should get output similar to this:

php-8.2-simplexml-8.2.17-r0
php-8.2-simplexml-config-8.2.17-r0
php-8.2-xml-8.2.17-r0
php-8.2-xml-config-8.2.17-r0
php-8.2-xmlreader-8.2.17-r0
php-8.2-xmlreader-config-8.2.17-r0
php-8.2-xmlwriter-8.2.17-r0
php-8.2-xmlwriter-config-8.2.17-r0
php-simplexml-8.2.11-r1
php-xml-8.2.11-r1
php-xmlreader-8.2.11-r1
php-xmlwriter-8.2.11-r1

Searching which package has a command

To search in which package you can find a command, you can use the syntax apk search cmd:command-name. For instance, if you want to discover which package has the command useradd, you can use:

apk search cmd:useradd

You’ll get output indicating that the shadow package has the command you are looking for.

shadow-4.15.1-r0

Searching for package dependencies

To check for package dependencies, you can use the syntax apk search -R info package-name. For example, to search which packages are listed as dependencies for the shadow package that we’ve seen in the previous section, you can run:

apk -R info shadow

And this will give you a list of dependencies for each version of the shadow package currently available:

...
shadow-4.15.1-r0 depends on:
so:ld-linux-x86-64.so.2
so:libbsd.so.0
so:libc.so.6
so:libcrypt.so.1
so:libpam.so.0
so:libpam_misc.so.0

Searching for packages that include a shared object

To search which packages include a shared object, you can use the syntax apk search so:shared-library. As an example, if you want to check which packages include the libxml2 shared library, you can run something like:

apk search so:libxml2.so*

And this should give you output indicating that this shared object is included within the libxml2-2.12.6-r0 package.

For detailed information about apk options and flags when searching for packages, check the official documentation.

Resources to Learn More

Our Getting Started Guides have detailed examples for different language ecosystems and stacks. Make sure to also check image-specific information in our Chainguard Images Directory.

If you can’t find an image that is a good match for your use case, or if your build has dependencies that cannot be met with the regular catalog, get in touch with us for alternative options.