Avoid explicit PR HEAD ref in workflow

Author: SimplyDanny

.github/workflows/docker.yml

@@ -50,7 +50,7 @@ jobs:
       if: github.event_name == 'pull_request'
       run: |
         {
-          echo "CHECKOUT_REF=${{ github.event.pull_request.head.ref }}"
+          echo "CHECKOUT_REF=''"
           echo "DOCKER_TAG=pr-${{ github.event.pull_request.number }}"
           echo "OUTPUT_TYPE=type=local,dest=artifacts"
         } >> "$GITHUB_ENV"