Add Docker image cleanup container to compose stack

Adds a lightweight docker:cli sidecar that prunes dangling images
every 24 hours to prevent untagged image buildup from redeployments.

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

docker-compose.yml

@@ -48,6 +48,14 @@ services:
     depends_on:
       - api
 
+  docker-cleanup:
+    image: docker:cli
+    container_name: purrse-docker-cleanup
+    restart: unless-stopped
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+    entrypoint: ["/bin/sh", "-c", "while true; do sleep 86400; docker image prune -f; done"]
+
 volumes:
   purrse-db-data:
   purrse-plugins: