container: add global vars for image-build tasks
    
    composes were failing because the 2nd image build task
    for the minimal image (copied from fedora.conf) didn't
    have some variables defined. In fedora.conf the global_*
    vars are defined, so let's do that here too.
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>
    (cherry picked from commit 6a859299436cd1e664136c451121465a5cb689b2)