TiDB Labs Nixpacks reference for LLMs

Current state
- Nixpacks is not the production build or deploy path for this repo.
- The repo ships with Dockerfiles and Ansible deployment instead.

Canonical build/deploy anchors
- `frontend/Dockerfile`
- `backend/Dockerfile`
- `deployment/roles/labs_service/files/docker-compose.yml`
- `deployment/playbooks/**`

Guidance
- Do not assume Nixpacks conventions are authoritative here.
- If a task mentions Nixpacks, treat it as an exception that must be reconciled
  with the existing Docker and Ansible flow.