#7920: docs: Add Docker Hardened Images (DHI) guide

## Summary Adds documentation for using Docker Hardened Images (DHI) with OpenClaw. **🤖 AI-Assisted:** This PR was created with AI assistance (fully tested and verified). ## What's included - New guide: `docs/install/docker-hardened-images.md` - Link from main Docker installation guide - Migration examples for gateway and sandbox images - Security benefits and considerations - Automated rebuild script ## Why DHI? Docker Hardened Images provide: - Reduced attack surface (distroless approach) - Fewer vulnerabilities vs standard images - Cryptographic signing and SBOMs - Maintained by Docker with continuous updates ## Testing - Verified Node.js 22 availability in DHI catalog - Tested multi-stage build approach locally - Confirmed compatibility with OpenClaw architecture - Built and ran gateway successfully with DHI images ## Notes - DHI is optional - standard images continue to work - Focused on documentation, no code changes - Based on DHI official documentation and hands-on testing