Attempt to make the OCI workflow behave reasonably for PRs

When repo secrets are not available to the actions run, the docker
image is built, but not published

(cherry picked from commit 805ed8f)
(cherry picked from commit e2efd19)

Author: HoloRin

.github/workflows/oci.yaml

@@ -103,7 +103,17 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-${{ matrix.image_tag_suffix }}-buildx-
 
+      - name: Check for Push Credentials
+        id: authorized
+        run: |
+          if [ -n "${{ secrets.DOCKERHUB_USERNAME }}" ]; then
+            echo "::set-output name=PUSH::true"
+          else
+            echo "::set-output name=PUSH::false"
+          fi
+
       - name: Login to DockerHub
+        if: steps.authorized.outputs.PUSH
         uses: docker/login-action@v1
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
@@ -124,7 +134,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           context: packaging/docker-image
-          push: true
+          push: ${{ steps.authorized.outputs.PUSH }}
           tags: |
             pivotalrabbitmq/rabbitmq:${{ steps.compute-tags.outputs.TAG_1 }}
             pivotalrabbitmq/rabbitmq:${{ steps.compute-tags.outputs.TAG_2 }}