Still trying to package staytus, but bundler fails with 
> There was an error while trying to write to
> `/nix/store/zmyqdcwk1h2pjvwq5pm5hmsxpswihfvl-gemfile-and-lockfile/.bundle/config`.
> It is likely that you need to grant write permissions for that path.
Am I missing something obvious? default.nix is here: https://pastebin.com/umXtHZMd