refactor(entrypoint): improve privilege dropping and config initialization

- Updated entrypoint.sh to initialize the on-disk config only if it does not already exist, preserving user edits across restarts.
- Enhanced privilege dropping logic to ensure the container does not run as root if the privilege drop fails.
- Clarified comments in Dockerfile and entrypoint.sh for better understanding of the privilege management process.
- Updated README.md to include a note about persistent disks requiring a paid service on Render.
- Adjusted render.yaml to clarify the Docker command behavior and added a note regarding auto-deploy settings.

README.md

@@ -52,6 +52,8 @@
 
 Deploy nanobot's gateway and bundled WebUI as a single web service with persistent memory. Render reads [`render.yaml`](./render.yaml) and prompts for two secrets on deploy: `ANTHROPIC_API_KEY` and `NANOBOT_WEB_TOKEN` (the password that gates the public WebUI — generate a strong random value, e.g. `openssl rand -hex 32`).
 
+> **Note:** The blueprint attaches a persistent disk so sessions, memory, and WebUI history survive restarts. Persistent disks require a paid service (they are not available on Render's free tier).
+
 [![Deploy to Render](https://render.com/images/deploy-to-render-button.svg)](https://render.com/deploy?repo=https://github.com/HKUDS/nanobot)
 
 ## What can nanobot do?