Rebuild the contributor image every friday morning

This was tried in the past but failed for some of our images, so this is
the first check to make this work again.

Author: Michael Vasseur

.github/workflows/build-contributor-container-release.yml

@@ -3,6 +3,8 @@ name: 'Build Contributor container (Release)'
 # Loosely based on: https://docs.docker.com/build/ci/github-actions/push-multi-registries/
 
 on:
+  schedule:
+    - cron: "5 4 * * 5"
   push:
     branches:
       - main