## Summary
- **The `--docker` flag was a no-op stub** — `renderDocker` called the same local `executeRenderJob` as `renderLocal`, no container was ever launched
- Now `renderDocker` generates a Dockerfile, builds a versioned `hyperframes-renderer:<version>` image with Chrome/FFmpeg/fonts/chrome-headless-shell, and runs the render inside a container
- Image is cached per CLI version — first render builds (~2 min), subsequent renders reuse it
- Forces `linux/amd64` platform since chrome-headless-shell has no ARM Linux binary
- Uses `execFileSync` (array form) throughout to prevent shell injection
- Forwards `--quiet`, `--gpu`, and render config flags into the container
- `Dockerfile.render` added as a reference for manual builds
## Test plan
- [x] `hyperframes render --docker` builds image and produces valid MP4
- [x] Second run reuses cached image (no rebuild)
- [x] `--quiet` suppresses container output while keeping stderr for errors
- [x] Typecheck, lint, format all pass
- [ ] Verify `--gpu` with `--docker` on a machine with GPU access
🤖 Generated with [Claude Code](https://claude.com/claude-code)
