The name gitlab-runner clashes with a component of Gitlab CI with the same name and only confuses people. It's now called gitlab-bundle and a convenience-script gitlab-rake for easier invocation of rake tasks was added. This was the primary use case of gitlab-runner.
To build the manual, you need Nix installed on your system (no need
for NixOS). To install Nix, follow the instructions at
https://nixos.org/nix/download.html
When you have Nix on your system, in the root directory of the project
(i.e., `nixpkgs`), run:
nix-build nixos/release.nix -A manual.x86_64-linux
When this command successfully finishes, it will tell you where the
manual got generated.