ci: tag with branch name if not default branch

This should prevent failed runs like https://github.com/NodeBB/NodeBB/actions/runs/4959641483

.github/workflows/docker.yml

@@ -47,6 +47,7 @@ jobs:
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}.x
             type=raw,value=latest,enable={{is_default_branch}}
+            type=ref,event=branch,enable={{#if is_default_branch}}false{{else}}true{{/if}}
 
       - name: Build and push Docker images
         uses: docker/build-push-action@v4