ci: protect Docker 'stable' tag

This commit introduces a safeguard to ensure that the Docker image
tagged
as 'stable' is only pushed when a new tag is created on the stable
branch.
This prevents unintended updates to the stable Docker image, ensuring
consistency and reliability for users relying on the stable tag.

.github/workflows/docker.yaml

@@ -81,7 +81,7 @@ jobs:
             type=semver,pattern={{major}}.{{minor}},prefix=v
             type=raw,value=latest,enable={{is_default_branch}}
             type=raw,value=${{ github.event.pull_request.head.ref }}
-            type=raw,value=stable,enable=${{ startsWith(github.event.ref, 'refs/tags/v') }}
+            type=raw,value=stable,enable=${{ startsWith(github.event.ref, 'refs/tags/v') && is_default_branch }}
 
       - name: Build and push livepeer docker image
         uses: docker/build-push-action@v5