docs: clarify UPAAS_DATA_DIR default is for local dev only

The ./data default comes from Go code and works for local development.
For Docker deployments, an absolute path should be used.
Updated config table to make this distinction clear.

README.md

@@ -157,7 +157,7 @@ Environment variables:
 | Variable | Description | Default |
 |----------|-------------|---------|
 | `PORT` | HTTP listen port | 8080 |
-| `UPAAS_DATA_DIR` | Data directory for SQLite and keys | ./data |
+| `UPAAS_DATA_DIR` | Data directory for SQLite and keys | `./data` (local dev only — use absolute path for Docker) |
 | `UPAAS_HOST_DATA_DIR` | Host path for DATA_DIR (when running in container) | *(none — must be set to an absolute path)* |
 | `UPAAS_DOCKER_HOST` | Docker socket path | unix:///var/run/docker.sock |
 | `DEBUG` | Enable debug logging | false |