Merge pull request #6751 from mchelen/patch-3

missing `sudo`

docs/sources/userguide/dockerimages.md

@@ -380,7 +380,7 @@ containers](
 
 Let's delete the `training/sinatra` image as we don't need it anymore.
 
-    $ docker rmi training/sinatra
+    $ sudo docker rmi training/sinatra
     Untagged: training/sinatra:latest
     Deleted: 5bc342fa0b91cabf65246837015197eecfa24b2213ed6a51a8974ae250fedd8d
     Deleted: ed0fffdcdae5eb2c3a55549857a8be7fc8bc4241fb19ad714364cbfd7a56b22f