ci: switch to Gitea Container Registry for reliable deploys

Previous pipeline built images locally via Portainer Docker API,
but Docker layer caching produced identical images. Now:
- Build with nocache=1
- Push to Gitea registry (git.home.rm-warpstation.de)
- Compose uses image: from registry instead of build:
- Redeploy with pullImage: true forces fresh container

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

docker-compose.yml

@@ -1,8 +1,6 @@
 services:
   tippspiel:
-    build:
-      context: .
-      dockerfile: Dockerfile
+    image: git.home.rm-warpstation.de/mwf975_git/tippspiel:latest
     container_name: wm2026-tippspiel
     restart: unless-stopped
     ports: