ci(pr): checkout base_ref

Author: ttshivers

.github/workflows/official-pr.yml

@@ -22,7 +22,7 @@ jobs:
         uses: actions/checkout@v2
         with:
           path: docker-node
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: ${{ github.base_ref }}
           fetch-depth: 50
 
       - name: Checkout the official-images repo