Add ppc64le and s390x support to bootc-image-builder

[pcdubs]

The fedora-bootc images include all four architectures supported by Fedora, however bootc-image-builder currently supports x86_64 and aarch64 only.

Opening this to request/discuss adding support for ppc64le and s390x .

[mvo5]

Could you please elaborate what is not working? We have a smoke test in https://github.com/osbuild/bootc-image-builder/blob/main/test/testcases.py#L102 that will generate manifests for ppc64le/s390x which should be supported via --target-arch. Could you please paste the failing commandline?

[ondrejbudai]

Is this issue about us not providing bib container images for these arches on quay?

[cgwalters]

Yes I think this is about the current disparity in build systems between the two. I am not aware of any reason not to update the pipeline here to build for all 4, right?