feat: add Dockerfile, docker-compose, and Docker documentation

[liuxiaopai-ai]

Closes #7

What's included

1. Multi-stage Dockerfile

• Stage 1 (deps): node:22-alpine, installs pnpm via corepack, copies workspace manifests, runs pnpm install --frozen-lockfile
• Stage 2 (build): Copies source code, runs pnpm build (builds only apps/webclaw via the root script)
• Stage 3 (runtime): Clean node:22-alpine with only the Nitro .output/ server bundle — minimal image size

2. docker-compose.yml

• Webclaw service with port mapping 3000:3000
• Environment variables sourced from .env with sensible defaults
• restart: unless-stopped policy

3. .dockerignore

• Excludes node_modules, .git, dist, .output, env files, IDE configs, etc.

4. README update

• Added a Docker section after the existing Setup section
• Documents both docker compose up --build and manual docker build/docker run
• Includes host.docker.internal hint for connecting to a host-side gateway

Key design decisions

• pnpm workspace-aware: Copies both apps/webclaw/package.json and apps/landing/package.json so workspace resolution succeeds, but only builds webclaw
• Corepack for pnpm: Uses corepack prepare pnpm@9.15.4 matching the repo's packageManager field
• Minimal runtime: Final stage contains only the self-contained Nitro server output — no node_modules, no source code
• Environment variables: CLAWDBOT_GATEWAY_URL, CLAWDBOT_GATEWAY_TOKEN, CLAWDBOT_GATEWAY_PASSWORD configurable at runtime

[ibelick]

@liuxiaopai-ai thanks for the PR. Does the container start and serve the app using only .output (no node_modules) on a fresh docker run?