Add container garbage collection to cleanup page

doc/raketasks/cleanup.md

@@ -179,3 +179,8 @@ sudo gitlab-rake gitlab:cleanup:sessions:active_sessions_lookup_keys
 # installation from source
 bundle exec rake gitlab:cleanup:sessions:active_sessions_lookup_keys RAILS_ENV=production
 ```
+
+## Container Registry garbage collection
+
+Container Registry can use considerable amounts of disk space. To clear up
+unused layers, the registry includes a [garbage collect command](../administration/packages/container_registry.md#container-registry-garbage-collection).