Support changes in /etc/os-release without requiring new OS definitions

[p5]

Do we want to add manifests for all custom images? I can imageing the list becoming very long. Or should there be some way to pass it in to the build?

Probably a bit of both; I see no real necessity to add these definitions if they're going to be symlinks to centos-9 so perhaps something nice would be possible there so these types of PRs are necessary only for actual new distros.

#771 (comment)

When an image changes the /etc/os-release file, they are no longer able to use bootc-image-builder since they don't have a definition inside the osbuild/bootc-image-builder repo.

As suggested by @supakeen, it would be great to somehow allow the user to say "I am building an image based on C10s" so they don't need to fork or commit these symlinks to this repo.
If they are in fact creating a whole new distro, then it seems okay to require a PR here to centralise things. It should be far less frequent.

[hanthor]

https://gitlab.com/fedora/bootc/base-images/-/blob/main/fedora-bootc-config.json?ref_type=heads

Not sure why we are specifying the bib image in the base image but cannot specify the base image in bib?

[ondrejbudai]

Would something like --iso-def centos-10 help?

Alternatively, you can build a layer on top of bootc-image-builder with your custom defs, but I guess that having "native support" would be better.

[cgwalters]

I think the correct thing to do here is to honor ID_LIKE in the os-release file.