doc: add comment

templates/gitlab-ci-docker.yml

@@ -269,6 +269,9 @@ stages:
     done
   }
 
+  # Cf https://docs.docker.com/engine/reference/commandline/tag/
+  # > A tag name must be valid ASCII and may contain lowercase and uppercase letters, digits, underscores, periods and dashes.
+  # > A tag name may not start with a period or a dash and may contain a maximum of 128 characters.
   function normalize_image() {
     imgprefix=${1%%:*}
     tagonly=${1#*:}