Set docker tag in consistent manner (#2096)

b/316402019

.github/actions/docker/action.yaml

@@ -43,12 +43,22 @@ runs:
     # We need to set docker tag properly for pull requests.  In those scenarios where no docker related files
     # were changed we need to use an existing image (e.g. main).  In cases where docker image is rebuilt we have
     # to use tag generated by the image build.
+    - name: Retrieve Docker metadata for PR
+      id: pr-meta
+      uses: docker/metadata-action@507c2f2dc502c992ad446e3d7a5dfbe311567a96 # v4.3.0
+      with:
+        images: ${{env.REGISTRY}}/${{github.repository}}/${{inputs.docker_image}}
+        tags: type=raw,value=${{ github.base_ref }}
     - name: Set Docker Tag
       id: set-docker-tag-presubmit-non-fork
       env:
         REPO: ${{ github.repository }}
       if: ${{ (steps.changed-files.outputs.any_changed == 'false') && (github.event_name == 'pull_request') }}
-      run: echo "DOCKER_TAG=ghcr.io/${REPO}/${{inputs.docker_image}}:${GITHUB_BASE_REF%.1+}" >> $GITHUB_ENV
+      run: |
+        set -x
+        docker_tag="${{ steps.pr-meta.outputs.tags }}"
+        docker_tag="${docker_tag%.1[+,-]}"
+        echo "DOCKER_TAG=${docker_tag}" >> $GITHUB_ENV
       shell: bash
     - name: Set up Cloud SDK
       if: ${{ (steps.changed-files.outputs.any_changed == 'true') && (github.event_name == 'pull_request') && (github.event.pull_request.head.repo.fork) }}