Force connection of stdin and stderr in the run command.

If stdin is closed, which is the case when pluginhook is called from an
interactive session, "docker run" does not connect stdin and stderr by default.
Fixes #323

plugins/00_dokku-standard/commands

@@ -51,7 +51,7 @@ case "$1" in
         exit 1
     fi
     shift 2
-    docker run -i -t $IMAGE /exec "$@"
+    docker run -i -t -a stdout -a stderr $IMAGE /exec "$@"
     ;;
 
   url)