Bump docker/build-push-action from 5 to 6

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 5 to 6.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](https://github.com/docker/build-push-action/compare/v5...v6)

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

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

.github/workflows/docker.yml

@@ -32,7 +32,7 @@ jobs:
         with:
           username: qutebrowser
           password: ${{ secrets.DOCKER_TOKEN }}
-      - uses: docker/build-push-action@v5
+      - uses: docker/build-push-action@v6
         with:
           file: scripts/dev/ci/docker/Dockerfile
           context: .