test: CI smoke test — Docker image builds and starts healthy

[icebear0828]

Goal

Add a GitHub Actions workflow that verifies the Docker image builds and the container starts healthy.

Acceptance criteria

• New workflow file: .github/workflows/docker-smoke-test.yml
• Triggers on push to master and on pull_request
• Steps:
  1. docker build -t codex-proxy-test .
  2. Run container on default port 8080, wait for healthcheck (up to 30s), curl /health returns 200
  3. Cleanup, then test custom port 8090 by mounting a modified config/default.yaml (use sed to change port: 8080 → port: 8090)
  4. Verify /health returns 200 on 8090
• Runner: ubuntu-latest

Context

• Dockerfile is at repo root
• Healthcheck script: docker-healthcheck.sh (reads port from config/default.yaml)
• Health endpoint: GET /health
• Port is configured in YAML config, NOT via environment variable

Constraints — read before writing code

• Package manager: npm — do NOT introduce pnpm, yarn, or any lock file other than package-lock.json
• Branch name: master — this repo's default branch is master, not main
• Do NOT modify .gitignore — tests/ is intentionally gitignored
• Do NOT modify existing dependencies — no version bumps, no downgrades, no moving deps between dependencies/devDependencies
• Do NOT add new dependencies unless absolutely necessary for this workflow (a pure shell/docker workflow should need none)
• TypeScript: no any — if you write TS, use concrete types, unknown, or generics
• This is a standalone workflow — do NOT combine with unit tests or other smoke tests. Keep it focused on Docker only.

[icebear0828]

Additional constraints (based on failed PRs #130, #136, #139)

Previous attempts kept introducing unrelated changes. Adding explicit guardrails:

• Do NOT modify any existing file — only create .github/workflows/docker-smoke-test.yml. No package.json, package-lock.json, web/, config/, or anything else.
• On healthcheck or curl failure, always dump docker logs <container> before exit 1 — otherwise CI failures are impossible to debug.
• Do NOT combine this with other smoke tests (Electron, Web) — keep it a single focused workflow.

[icebear0828]

Closing to rewrite with stricter constraints.