Update Dockerfile to explicitly install python2-gunicorn
    
    This seems to resolve the strange behaviour, where subsequent
    requests for decisions reported different versions of gunicorn
    in Server (gunicorn/19.7.1 and gunicorn/19.6.0) .
    
    Not entierly sure if the fix is due to switching to Fedora 27,
    or the explicit python2-gunicorn install, though.
    
    This does not solve the issue that ~40% of the decision requests
    will return "404 NOT FOUND" .
    
    Signed-off-by: Hunor Csomortáni <csomh@redhat.com>