Fix Docker model cache path for cloud deployments

[toubatbrian]

Summary

Fixes Node agent cloud deployments where pnpm download-files succeeds during Docker build, but the worker later fails to find the LiveKit turn detector model at runtime.

The multi-stage Dockerfile runs download-files in the build stage as root, which caused HuggingFace/model files to be cached under /root/.cache/huggingface. Since the final image only copies /app, those files were missing at runtime when the app runs as appuser with home /app.

This sets HOME=/app in the shared base stage so build-time downloads and runtime model lookup use the same cache location under /app.

Test plan

• Built the Docker image.
• Verified pnpm download-files writes model files under /app/.cache/huggingface.
• Verified the runtime container starts without File not found in cache for livekit/turn-detector.