Simplify images cleanup too. Reviewed containers cleanup.

- Images cleanup is a common practice taken from
  https://docs.docker.com/reference/commandline/cli/#filtering_1

dokku

@@ -122,9 +122,9 @@ case "$1" in
 
   cleanup)
     # delete all non-running container
-    docker ps -a -f "status=exited" -q | xargs docker rm &> /dev/null &
+    docker rm $(docker ps -a -f "status=exited" -q) &> /dev/null &
     # delete unused images
-    docker images | grep '<none>' | awk '{print $3}' | xargs docker rmi &> /dev/null &
+    docker rmi $(sudo docker images -f "dangling=true" -q) &> /dev/null &
     ;;
 
   plugins)