Add automatic repository owner to image naming

- Use github.repository_owner in workflow
- Image format: REGISTRY_URL/owner/APP_NAME:latest
- Works automatically with Gitea registry
- Template-friendly: forks use their own owner
- Update documentation with new format

docker-compose.yml

@@ -2,7 +2,7 @@ version: '3.8'
 
 services:
   app:
-    image: ${REG}/${APP_NAME}:latest
+    image: ${REG}/${REPO_OWNER}/${APP_NAME}:latest
     container_name: ${APP_NAME}
     restart: unless-stopped
     environment: