Fix(cpu/docker): install Rust for appuser, set PATH, and increase uv timeouts to stabilize build

[nRanzo]

Summary

This PR fixes intermittent CPU image build failures by ensuring Rust is available to the non-root user during dependency installation and by increasing network timeouts for large wheels. It stabilizes uv sync on Apple Silicon and similar arm64 environments.

What changed

• Dockerfile
  • Install Rust via rustup as the non-root appuser (so Rust is on PATH during uv sync).
  • Export PATH to include /home/appuser/.cargo/bin and /app/.venv/bin before dependency installation.
  • Add UV_HTTP_TIMEOUT=120 and UV_HTTP_RETRIES=3 to reduce transient network timeouts when fetching large wheels (e.g., numpy, torch).

No runtime behavior change; only build reliability improvements.

Why

• sudachipy (pulled via pyopenjtalk-plus → misaki[ja]) requires a Rust compiler to build on arm64. When uv sync runs under appuser, Rust wasn’t available on PATH, causing:
  • error: can’t find Rust compiler
• Large wheels occasionally failed with network timeouts during extraction. Increasing uv HTTP timeouts and retries mitigates this.

How I tested

• Built locally on macOS (Apple Silicon):
  • docker compose up --build in docker/cpu
  • Image built successfully; uv sync completed without Rust or numpy timeouts.
• Container started, downloaded model, warmed up, and served:
  • Web UI: http://localhost:8880/web/
  • OpenAI-compatible API: http://localhost:8880/v1
  • Verified logs show model warmup and server ready.

Risk/impact

• Low. Changes are limited to the CPU Dockerfile build steps and environment variables.
• GPU paths/images are unaffected.

Notes for reviewers

• Installing Rust as appuser is necessary because uv sync runs as that user; installing only as root leaves Rust off PATH for appuser.
• Timeout values are conservative and only affect build-time network operations (via uv).

Checklist

• CPU image builds reliably on arm64 (macOS) with docker compose up --build.
• Container starts and serves /web and /v1 endpoints.
• No breaking changes to runtime environment or API.

Related

• Fixes build errors like “can’t find Rust compiler” during sudachipy build.
• Mitigates uv network timeouts for large wheels.