Figure out how to build, ship and use sysexts using containers

[travier]

Right now, we are building most of the sysexts using both the "single EROFS" file approach via a justfile and the "container layer" approach using a Containerfile and a classic container build process.

To be able to use the container layers with bootc (containers/bootc#7), we will need to teach bootc how to manage those container images and "convert" them to sysexts.

Using https://github.com/containers/composefs-rs could be part of the solution once this is integrated into bootc.

In the mean time, we could also use the logically-bound container logic in bootc to pull those container images and then add some logic that converts them to "classic" single-file erofs sysexts.

We could also consider changing the build pipeline for the container flow to directly extract the last layer produced by the container build and push only that layer (and not the full image like we do right now) to the registry.

See also: https://github.com/travier/fedora-sysexts/blob/main/README.containers.md