Add Gitea Actions CI pipeline for Docker builds

- Builds and pushes Docker image on main branch commits
- Tags with both latest and commit SHA
- Uses build cache for faster builds

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.gitea/workflows/docker.yml

@@ -0,0 +1,34 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Docker Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ secrets.DOCKER_REGISTRY }}
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: |
+            ${{ secrets.DOCKER_REGISTRY }}/calendar:latest
+            ${{ secrets.DOCKER_REGISTRY }}/calendar:${{ github.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max