fix(docker): make instance directory configurable and writable

- Add INSTANCE_BASE_PATH config option (defaults to /data/instances)
- Update docker.py to use configured path instead of hardcoded 'data/instances'
- Update Dockerfile to create /data/instances and chown to appuser
- Add instance_data volume to docker-compose.traefik.yml and docker-compose.yml
- Set INSTANCE_BASE_PATH env var in both compose files

This fixes the PermissionError when creating tool instances because
appuser can now write to /data/instances.

apps/api/src/services/docker.py

@@ -23,16 +23,19 @@ def render_compose_template(template: str, variables: dict[str, Any]) -> str:
     return result
 
 
-def ensure_instance_directory(instance_id: str, base_path: str = "data/instances") -> str:
+def ensure_instance_directory(instance_id: str, base_path: str | None = None) -> str:
     """Create and return the instance directory path.
 
     Args:
         instance_id: Unique instance identifier
-        base_path: Base directory for all instances
+        base_path: Base directory for all instances (defaults to Settings.instance_base_path)
 
     Returns:
         Absolute path to instance directory
     """
+    if base_path is None:
+        from src.config import Settings
+        base_path = Settings().instance_base_path
     instance_dir = Path(base_path) / instance_id
     instance_dir.mkdir(parents=True, exist_ok=True)
     return str(instance_dir.absolute())