Slightly improve "cleanup".

- Use docker ps to filter containers by status to avoid building
  such a long pipe.

dokku

@@ -122,7 +122,7 @@ case "$1" in
 
   cleanup)
     # delete all non-running container
-    docker ps -a | grep 'Exit' | awk '{print $1}' | xargs docker rm &> /dev/null &
+    docker ps -a -f "status=exited" -q | xargs docker rm &> /dev/null &
     # delete unused images
     docker images | grep '<none>' | awk '{print $3}' | xargs docker rmi &> /dev/null &
     ;;