feat(commitlint): users can configure image with `$COMMITLINT_IMAGE`

Users can override the variable in their `.gitlab-ci.yml` to select
another docker image.

templates/Lint/Commitlint.yaml

@@ -4,7 +4,9 @@
 commitlint:
   stage: lint
   extends: .not-on-stable
-  image: 'hub.eole.education/eole/commitlint:latest'
+  image: "$COMMITLINT_IMAGE"
+  variables:
+    COMMITLINT_IMAGE: 'hub.eole.education/eole/commitlint:latest'
   script:
     - 'git fetch --all'
     # Set default commit hashes for `--from` and `--to`