fix(docker): add docker group to API container for socket access

The API container needs access to /var/run/docker.sock to run
docker compose commands for tool instances. Add group_add to
match the host's docker GID.

Error was:
permission denied while trying to connect to the docker API at unix:///var/run/docker.sock

docker-compose.traefik.yml

@@ -102,6 +102,8 @@ services:
       - avatar_uploads:/app/uploads
       - /var/run/docker.sock:/var/run/docker.sock
       - cloudflared_config:/etc/cloudflared
+    group_add:
+      - ${DOCKER_GID:-999}
     depends_on:
       postgres:
         condition: service_healthy