remove dangling docker images at once

docker/README.md

@@ -34,6 +34,10 @@
   $ docker-compose stop
   $ docker load -i <image-name>.tar
   $ docker-compose up -d --build
+  
+  // delete all dangling images at once
+  // -q return only the image unique id
+  $ docker rmi $(docker images -q --filter "dangling=true")
   ```