build(deps): bump docker/metadata-action from 5 to 6

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

---
updated-dependencies:
- dependency-name: docker/metadata-action
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/container-image.yml

@@ -15,7 +15,7 @@ jobs:
     - id: meta
       # > "automatic" tag management and OCI Image Format Spec… for labels
       # https://web.archive.org/web/20250103123053/https://docs.docker.com/build/ci/github-actions/manage-tags-labels/
-      uses: docker/metadata-action@v5
+      uses: docker/metadata-action@v6
       with:
         # see comment on `push: true` below
         images: ghcr.io/${{ github.repository_owner }}/postfix