diff --git a/templates/gitlab-ci-docker.yml b/templates/gitlab-ci-docker.yml
index 7255313720f3f32de1c0470e9fe06c20153c508f..fd477835df1ca408ee3b79e1dfd89b0c67c1007e 100644
--- a/templates/gitlab-ci-docker.yml
+++ b/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#*:}