ci: add branchname to docker tag

Author: HilkopterBob

.github/workflows/unstable-build-docker-container.yml

@@ -18,4 +18,4 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           push: true
-          tags: hilkopterbob/packagelock:unstable-${{ env.BRANCH }}
+          tags: hilkopterbob/packagelock:unstable-${{ github.head_ref || github.ref_name }}