[codex] Add container deployment guide

[pq198363-ops]

Closes #102

Summary

• Added a multi-stage production Dockerfile.
• Added a root .dockerignore that excludes local dependencies, build output, git metadata, env files, logs, docs, and tests from the image context.
• Documented Docker build/run commands, /health verification, non-root runtime behavior, environment variable usage, and SIGTERM shutdown expectations in the README.

Validation

RED evidence:

• Test-Path .\Dockerfile returned False.
• Test-Path .\.dockerignore returned False.

GREEN evidence:

• Test-Path .\Dockerfile returned True.
• Test-Path .\.dockerignore returned True.
• npx prettier --check README.md
• npm run lint
• git diff --check
• Cleaned dist/, then ran npm run build
• Windows test command: $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js

Result: full clean test run passed, 89 tests across 10 suites.

Docker note

Docker CLI is not installed in this Windows environment, so I could not run a local docker build or container smoke test here. The Dockerfile is scoped to the existing npm ci, npm run build, production dependency install, non-root node user, EXPOSE 3001, STOPSIGNAL SIGTERM, and a Node-based /health healthcheck.