build(deps): bump docker/build-push-action from 6.13.0 to 6.15.0

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 6.13.0 to 6.15.0.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](https://github.com/docker/build-push-action/compare/ca877d9245402d1537745e0e356eab47c3520991...471d1dc4e07e5cdedd4c2171150001c434f0b7a4

)

---
updated-dependencies:
- dependency-name: docker/build-push-action
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build-container.yml

@@ -39,7 +39,7 @@ jobs:
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
       - name: Build and push Docker image
-        uses: docker/build-push-action@ca877d9245402d1537745e0e356eab47c3520991
+        uses: docker/build-push-action@471d1dc4e07e5cdedd4c2171150001c434f0b7a4
         with:
           context: ./contrib/docker
           push: ${{ github.repository_owner == 'freifunk-gluon' && github.event_name == 'push' }}