feat: Docker mode improvements

[nmhjklnm]

Summary

The current cac docker mode works but needs improvements.

Areas

• Image update strategy: currently a fixed Dockerfile, needs versioned builds or auto-rebuild
• Multi-container: different environments in different containers
• Config sync: auto-sync host cac config into container
• sing-box config: auto-generate optimized configuration
• GPU passthrough: for users running local models alongside Claude Code

Current state

cac docker setup|start|enter|check|port|stop — basic lifecycle management with sing-box TUN networking.