This is a demonstration of how cosign, apko and Melange can be combined to build an image with a custom application payload. It uses a set of GitHub Actions published in the Chainguard actions repository.
First, the relevant packages to support an application are built with Melange! In this case, we are using nginx as an example (a stripped down version of the nginx ingress packaging, which builds a few dozen dependencies).
You can see an example of this in the .melange.yaml file.
Next, the image is composed and published with apko. Melange stores its packages
in a local repository, which is consumed by apko. These packages are combined
with dependencies from the upstream Alpine Linux distribution, composed into an
OCI image, and published. An SBOM is generated and published along side the
image if apko 0.3 or newer is used.
Apko configures an s6 service bundle and arranges for it to be launched when a
container is started. This is similar to using s6-overlay with Docker, and is
considered a best practice so that zombie processes get reaped.
Finally, the image is signed using Cosign. You can see the .github/workflows/push.yaml
file for the details on how this works.
All of this is done in a declarative (and reproducible) way. Since it is declarative and reproducible, refreshing the image can be automated. See the distroless GitHub project for some examples of how this can be done.