Add option to force removing Dockerfile.stfubuntu

# Force removing stfubuntu docker image

if [ "$1" == "--force-stfubuntu" ]; then

    docker rmi `docker images -q stfubuntu` || exit 1

    docker rmi --force `docker images -q stfubuntu` || exit 1

fi

# Check and build stfubuntu image

if [ -z `docker images -q stfubuntu` ]; then