Add label interpolation into repos defined directly in the config

[lubomir]

This PR should enable this (assuming the last base image used in the Containerfile has a label compose-id:

  repos:
    - repoid: rhel
      baseurl: https://example.com/rhel/nightly/rhel/{compose-id}/compose/BaseOS/$basearch/os/
      varsFromContainerfile: Containerfile

If there are multiple repos, the information on where to get the labels from must be provided in each one separately, which is not great but doesn't break any backwards compatibility.

[msrb]

Works -- thanks ;)