Replace Docker-based builds with Nix flakes for reproducible builds.
Changes:
- Use nixos/nix:latest image with `nix` tag requirement
- Build Docker image with `nix build .#dockerImage`
- Add optional Cachix integration for binary caching
- Load image with `docker load < result`
- Keep existing deploy stage unchanged
Benefits:
- Reproducible builds across all environments
- Binary caching via Cachix (faster subsequent builds)
- All dependencies pinned in flake.lock
- Image size similar to Docker builds (~507 MB compressed)
Requirements:
- GitLab Runner with Nix installed and `nix` tag
- Docker socket mounted for loading/pushing images
- Optional: CACHIX_AUTH_TOKEN for binary caching
See docs/gitlab-runner-nix-setup.md for runner setup instructions.
The old Docker-based config is available in git history if rollback needed.
