Added digest to outputs

Author: SamTV12345

.github/workflows/docker.yml

@@ -107,6 +107,8 @@ jobs:
         if: github.event_name == 'push'
         uses: docker/build-push-action@v6
         with:
+          outputs:
+            - digest
           context: ./etherpad
           target: production
           platforms: linux/amd64,linux/arm64