container sync: use correct name for skopeo copy command
    
    Ah, the me from the last commit message was such a sweet, naive,
    hopeful summer child.
    
    Somehow I screwed up the final arg to `copy_image` completely
    when I rejigged this code into a function to avoid repetition.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>