feat(skills): add docker-security-audit skill for container hardening and vulnerability inspection

[huuanh20]

What this PR does

Adds a new production-ready skill \docker-security-audit\ under \docker-security-audit/SKILL.md:

• Guides Codex and AI agents on auditing Dockerfile & docker-compose configurations.
• Checks for non-root execution (\USER), base image version pinning, secret leaks in ENV/ARG, and multi-stage build layer optimization.