Fix Gitea action Docker build tag error

- Add fallback registry to prevent invalid tag format
- Make Docker login conditional on secrets being present
- Make push conditional on registry being configured
- Rename Docker image from calendar to runway

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

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

.gitea/workflows/docker.yml

@@ -16,6 +16,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
 
       - name: Login to Docker Registry
+        if: ${{ secrets.DOCKER_REGISTRY && secrets.DOCKER_USERNAME && secrets.DOCKER_PASSWORD }}
         uses: docker/login-action@v3
         with:
           registry: ${{ secrets.DOCKER_REGISTRY }}
@@ -26,9 +27,9 @@ jobs:
         uses: docker/build-push-action@v5
         with:
           context: .
-          push: true
+          push: ${{ secrets.DOCKER_REGISTRY != '' }}
           tags: |
-            ${{ secrets.DOCKER_REGISTRY }}/calendar:latest
-            ${{ secrets.DOCKER_REGISTRY }}/calendar:${{ github.sha }}
+            ${{ secrets.DOCKER_REGISTRY || 'localhost:5000' }}/runway:latest
+            ${{ secrets.DOCKER_REGISTRY || 'localhost:5000' }}/runway:${{ github.sha }}
           cache-from: type=gha
           cache-to: type=gha,mode=max