Actually, Guix and Nix aren't just package managers, they also provide a fully system configuration management tool complete with the ability to roll-back to earlier versions of the system. You don't need Ansible or Docker when you use Guix or Nix. The same framework for providing reproducible software builds is also used to provide reproducible system configuration.
See: http://www.gnu.org/software/guix/manual/html_node/Using-the-...