ci(docker): upgrade metadata-action v5 to v6

v5 uses Node.js 20 which is deprecated by GitHub Actions.
v6 supports Node.js 24.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: theothersideofgod

.github/workflows/docker.yaml

@@ -81,7 +81,7 @@ jobs:
 
       - name: Docker meta
         id: meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@v6
         with:
           images: ${{ env.REGISTRY }}/${{ github.repository_owner }}/${{ matrix.name }}-fn
           tags: |