fix: update ruff version requirement and use GITHUB_TOKEN for GHCR auth

- Change ruff>=0.15 to ruff>=0.6.0 (0.15 doesn't exist yet)
- Replace CPMP_GHCR_TOKEN with GITHUB_TOKEN for GHCR authentication
- GITHUB_TOKEN is automatically provided by GitHub Actions with correct permissions

.github/workflows/docker-build.yml

@@ -40,7 +40,7 @@ jobs:
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
-          password: ${{ secrets.CPMP_GHCR_TOKEN }}
+          password: ${{ secrets.GITHUB_TOKEN }}
 
       # Step 5: Figure out what tags to use
       - name: 🏷️ Extract metadata (tags, labels)