Add Docker support for Linux deployment

[TomGranot]

Replace Apple Container dependency with Docker to unlock Linux VPS deployment ($5-20/month vs $175-475/month Mac hosting).

Changes needed (~20 lines)

1. container-runner.ts: Replace spawn('container', ...) with configurable binary (default: docker)
2. index.ts: Replace container system status/start with docker info
3. build.sh: Replace container build with docker build
4. Dockerfile: Already OCI-compatible, zero changes needed
5. New: systemd unit file (Linux equivalent of launchd plist)

What this unlocks

• Hetzner: EUR 4.51/month
• DigitalOcean: $12/month
• Any Linux VPS: $5-20/month