Zuul: Add initial compose test (base only)
    
    Test composing the base variant for all PRs. We don't compose all
    variants yet as some fail in the Zuul infrastructure for an unknown
    reason.
    
    We only compose the base variant as it is the smallest and should give
    enough assurance about the builds while not delaying PR merging too
    much.
    
    See: https://pagure.io/workstation-ostree-config/pull-request/405