Migrate workflow to Gitea Actions

Move .github/workflows/ → .gitea/workflows/ and update secrets
to use REGISTRY_USER/REGISTRY_TOKEN with git.necloud.us registry.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker.yml

@@ -9,26 +9,23 @@ on:
       - main
 
 env:
-  REGISTRY: ghcr.io
+  REGISTRY: git.necloud.us
   IMAGE_NAME: ${{ github.repository }}
 
 jobs:
   build-and-push:
     runs-on: ubuntu-latest
-    permissions:
-      contents: read
-      packages: write
 
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Log in to GitHub Container Registry
+      - name: Log in to Gitea Container Registry
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          username: ${{ secrets.REGISTRY_USER }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Extract metadata
         id: meta