Fix act_runner: run as root for Docker socket access

group_add didn't resolve the permission issue. Running the runner
as root (user 0:0) is the standard approach for CI runners that
need Docker socket access on internal/single-user deployments.

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

docker-compose.yaml

@@ -66,8 +66,7 @@ services:
   act_runner:
     image: gitea/act_runner:latest
     restart: unless-stopped
-    group_add:
-      - "971"
+    user: "0:0"
     volumes:
       - act_runner_data:/data
       - /var/run/docker.sock:/var/run/docker.sock