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/config.py

@@ -49,6 +49,9 @@ class Settings(BaseSettings):
 
     # Repository storage
     repo_base_path: str = "/data/repos"
+    
+    # Tool instance storage
+    instance_base_path: str = "/data/instances"
 
     model_config = SettingsConfigDict(env_file=".env", extra="ignore", populate_by_name=True)