Fix SHORT in container-nightly.sh
    
    It's Fedora-Container, not Fedora-Docker. We changed it in 2018,
    but forgot to change this. Until now it only affected the
    send_fedmsg calls, which I don't think are even working anyway
    (I don't see any compose.* messages for anything besides rawhide,
    eln, and 41 ATM), but now it's breaking the metadata archive
    stuff.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>