ruby nix shell

config.nix

import ( builtins.fetchGit {
  name = "nixpkgs-2019-08-25";
  url = https://github.com/nixos/nixpkgs/;
  rev = "8d1510abfb592339e13ce8f6db6f29c1f8b72924";
}) {
  config = {
    allowUnfree = true;
  };
}

rubyenv.nix

{pkgs, bundlerEnv, ...}:

with pkgs;

bundlerEnv {
  inherit ruby_2_6;
  name      = "chronos-env";
  gemdir    = ../.;
  groups    = ["default" "development" "production" "test" "mover" "toolbox"];

  gemConfig.openssl = attrs: {
    buildInputs = [ openssl ];
  };

  gemConfig.pg = attrs: {
    buildInputs = [ postgresql_9_6 ];
  };

  gemConfig.nokogiri = attrs: {
    buildInputs = [ libiconv zlib ];
  };

  gemConfig.sassc = attrs: {
    buildInputs = [ libsass ];
    shellHook = ''
        export SASS_LIBSASS_PATH=${libsass}
    '';
  };
}

shell.nix

{ pkgs ? import ./nix/config.nix }:

let
  rubyenv = import ./nix/rubyenv.nix;

in pkgs.mkShell {
  buildInputs = with pkgs; [
    bundix
    bundler
    circleci-cli
    elasticsearch5
    libxml2
    nodejs
    postgresql_9_6
    rubyenv
    rubyenv.wrappedRuby
    solargraph
    yarn
  ];
}

I don't think I've seen that error before with this configuration, but I'd recommend taking a look at https://github.com/emptyflask/rails-nix for some more up-to-date code.