Implement app containerization for deployment testing

[MichaelSchmidle]

Generate and include user's app container in non-development environments.

Problem:

• light up production currently says "Start your app with npm run dev"
• This is dev workflow advice leaking into deployment testing
• Should deploy containerized app for local testing before remote deploy

Solution:

• Generate Dockerfile for user's app (if not exists) during light init
• Include app service in docker-compose.production.yml
• Build and start app container in light up <env> (non-dev)
• Remove "Start your app" messaging from non-dev output

Clear separation:

• light up → Dev mode: Proxy to localhost (no containerization)
• light up <env> → Deployment mode: Full stack with containerized app

Related: 001-T036-T040 from spec 001, #1 (blocks SSH deployment)

[MichaelSchmidle]

Closed by Spec 002 implementation (merged to main 2025-11-02).

All requirements from this issue were completed:

• ✅ Dockerfile support for user's app (user-provided via 'docker init')
• ✅ App service in docker-compose.deployment.yml
• ✅ App container builds and starts in deployment mode
• ✅ Removed 'Start your app with npm run dev' from deployment mode output
• ✅ Clear separation: light up (dev/proxy) vs light up (deployment/containerized)

See specs/002-dev-deploy-workflows/ for full implementation details.