tools/dockerfiles: Ensure merge requests can be built

jobs/tools/dockerfiles.yaml

@@ -6,6 +6,8 @@
     scm:
       - git:
           url: "{gitlab_url}/{gitlab_project_name}.git"
+          refspec: +refs/heads/*:refs/remotes/origin/*
+            +refs/merge-requests/*:refs/remotes/merge-requests/*
           wipe-workspace: false
     properties:
       - build-discarder: