feat: Add advanced Docker features blog post

[bobbyonmagic]

Closes #939

Adds comprehensive guide covering 5 advanced Docker features:

Content

1. BuildKit - Next-generation build engine with cache mounts and parallel builds
2. Multi-stage builds - Create minimal production images (1.2GB → 180MB)
3. Health checks - Verify container readiness for orchestrators
4. Init processes - Proper signal handling and zombie process prevention
5. Build secrets - Inject credentials without leaking them into image layers

Features

• 12-minute read with practical examples
• Production-ready Dockerfile combining all features
• Real-world impact metrics (build times, image sizes, deployment speed)
• Common mistakes section
• CI/CD integration examples (GitHub Actions, GitLab CI)
• OG images generated for social sharing

Testing

• ✅ All 4,571 tests passing
• ✅ OG images generated (SVG + PNG)
• ✅ Link checker validation ready

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