Add variable interpolation to repofile URLs

[lubomir]

Make it possible to interpolate image labels into the repofile URLs. This can be used to consume the exact same repos that were used to build the base image.

[msrb]

I think this will solve the problem described in https://github.com/konflux-ci/rpm-lockfile-prototype/issues/26 ;)

:+1:

[jpopelka]

I've just tried it in https://gitlab.com/redhat/rhel/containers/ubi10-init/-/merge_requests/192 and it works as expected. Thank you!