Test for socket being writable, not just existing
    
    The current system really only works if building the RPM as root
    (ewww) or if your user is in the `docker` group, which we discourage
    in favor of sudo.  I'm not sure how to plumb through sudo, but anyways
    it's more accurate to test writability.
    
    Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>