docs: add Docker deployment files and runner hosting guide
- Add Dockerfile for runner (Bun + Deno + ffmpeg)
- Add docker-compose.yml with volume mounts for keys
- Add HOSTING.md with deployment guides for Dokploy, Fly.io, etc.
- Add .dockerignore files
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>