ci: bump GitHub Actions to latest versions

- actions/checkout v1 → v6
- docker/setup-buildx-action v1 → v4
- docker/login-action v2 → v4

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hinaloe

.github/workflows/docker-publish.yml

@@ -49,12 +49,12 @@ jobs:
     if: github.event_name == 'push'
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v6
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v1
+        uses: docker/setup-buildx-action@v4
       - name: Login to GitHub Container Registry
-        uses: docker/login-action@v2
+        uses: docker/login-action@v4
         with:
           registry: ghcr.io
           username: ${{ github.actor }}