fix: install Docker CLI and mount socket for instance management

- Install docker-ce-cli and docker-compose-plugin in API Dockerfile
- Mount /var/run/docker.sock into API container
- Add appuser to docker group for socket permissions
- Fixes FileNotFoundError when deleting instances

docker-compose.traefik.yml

@@ -95,6 +95,7 @@ services:
       - repo_data:/data/repos
       - instance_data:/data/instances
       - avatar_uploads:/app/uploads
+      - /var/run/docker.sock:/var/run/docker.sock
     depends_on:
       postgres:
         condition: service_healthy