feat: add Dockerfile for openui-chat example

[zlplzp123wyt]

Fixes #303

Summary

Add a production-ready Dockerfile for the openui-chat example so users can build and run it with a single command:

docker build -t openui-chat .
docker run -p 3000:3000 -e OPENAI_API_KEY=sk-... openui-chat

Changes

• Dockerfile: Multi-stage build (builder + runner) that handles the pnpm monorepo workspace dependencies, builds the CLI + workspace packages, generates the system prompt, and produces a minimal production image
• next.config.ts: Enable output: "standalone" for efficient Docker builds
• .dockerignore: Exclude node_modules, .next, .git from build context

Architecture

The Dockerfile uses a two-stage approach:

1. Builder: Installs workspace deps, builds all required packages (@openuidev/cli, @openuidev/react-ui, @openuidev/react-headless, @openuidev/react-lang), generates the system prompt, and builds the Next.js app in standalone mode
2. Runner: Copies only the standalone output + static assets into a minimal Node.js image, runs as non-root user

Environment Variables

Variable	Description	Required
OPENAI_API_KEY	OpenAI API key for LLM calls	Yes
PORT	Server port (default: 3000)	No
HOSTNAME	Server hostname (default: 0.0.0.0)	No

[zahlekhan]

Thanks for your contribution. Your PR review is on hold since we have already assigned the issue to another contributor.
#305