Add Docker support — multi-stage Dockerfile and compose config

[officialasit]

Summary

Lets self-hosters run nodepad with a single docker compose up — no local Node toolchain needed. The container is stateless (notes and AI keys live in the browser's localStorage), so there's nothing to mount, back up, or configure on the server side.

• Multi-stage Dockerfile (deps → builder → runner) on node:22-alpine, non-root node user, ~313 MB image
• output: "standalone" in next.config.mjs for a minimal self-contained server bundle. This is ignored by Vercel's build adapter, so the existing Vercel deployment path is unaffected
• docker-compose.yml with init: true (tini as PID 1 for correct signal handling) and a node -e 'fetch(...)' healthcheck so the image doesn't need curl/wget
• HOST_PORT env var to override the host-side port mapping; the container always binds to 3000 internally. .env.example documents it
• .dockerignore mirroring .gitignore plus build artifacts and local-only files
• README: new "Run with Docker" subsection under Setup, with an explicit note about the single-replica constraint (see below)

Notes for reviewers

• Do not scale web beyond 1 replica. proxy.ts uses an in-memory Map as a rate limiter for /api/fetch-url, which assumes a single process. This is called out in both the compose file header and the README.
• No server-side secrets. The only env vars in .env.example control the container runtime (HOST_PORT). AI provider keys continue to live in localStorage and go directly to the provider.
• Standalone mode footgun handled: Next's output: "standalone" does not auto-copy public/ or .next/static; the runner stage copies them explicitly.

Test plan

• docker compose build succeeds on node:22-alpine
• docker compose up -d — container starts and healthcheck transitions to healthy
• GET / → 200, HTML served
• GET /icon.svg → 200 (confirms public/ was copied in standalone mode)
• cat /proc/1/comm inside container → docker-init (tini is PID 1, signal forwarding works)
• docker compose down — clean shutdown
• Reviewer smoke-test: add a note in the browser end-to-end (requires an AI provider key)

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