Containerize the Project

[yeevon]

Summary:
Containerize the Next.js app so it can be built and run consistently in Docker.

Acceptance Criteria:

1. Dockerfile added that builds a production image (install deps, build, run).
2. App runs in the container on port 3000, accessible from host (http://localhost:3000).
3. .dockerignore includes node_modules, .next, .git, and other common ignores.
4. Build (docker build ...) and run (docker run ...) commands documented in README.
5. Environment variables loaded via --env-file or equivalent; app boots without interactive prompts.