Add option to force removing Dockerfile.stfubuntu

docker/build.sh

@@ -7,7 +7,7 @@ set -vx
 
 # Force removing stfubuntu docker image
 if [ "$1" == "--force-stfubuntu" ]; then
-    if [ -z `docker images -q stfubuntu` ]; then
+    if [ -n `docker images -q stfubuntu` ]; then
         docker rmi --force `docker images -q stfubuntu` || exit 1
     fi
 fi