This branch prepares for allowing to have build roots from container sources. This is a bit of a shortcut PR that is mainly tailored to bootc-image-builder.

Alternatively we could change InstantiateManifest() to not take repos []rpmmd.RepoConfig but instead something like:

type RepoContainerStuff struct {
    Repos []rpmmd.RepoConfig
    Containers []container.SourceSpec
}

type ImageKind interface {
	Name() string
	InstantiateManifest(m *manifest.Manifest, repoContainer *RepoContainerStuff, runner runner.Runner, rng *rand.Rand) (*artifact.Artifact, error)
}

of course with a more sensible name. Happy to do this if that is the preferred way. I will move this out of draft (or close it) once I get feedback on this (or maybe there is a third option).

Once this is available, bib will use:

img := image.NewBootcDiskImage(containerSource)
...
_, err = img.InstantiateManifestFromContainers(&mf, containerSources, runner, rng)

Note that this is not sufficient for a full container based buildroot, the following is missing:

1. get EFI binaries from an alternative location (e.g. via stages(grub2): allow pulling efi binaries from alternative efi roots osbuild#1529 or bootupd)
2. create a second "bib-build" pipeline that uses the "bib" container as the buildroot and run "qemu-img" from there