Rails, Passenger, Nginx, Nix: uninitialized constant Bundler::Settings (NameError)

rails.log

Error: The application encountered the following error: uninitialized constant Bundler::Settings (NameError)
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler/settings.rb:6:in `<module:Bundler>'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler/settings.rb:5:in `<top (required)>'
    /nix/store/928i2cf1wizgm7dapmnk0m8ijrsjdj43-ruby-2.5.7/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:72:in `require'
    /nix/store/928i2cf1wizgm7dapmnk0m8ijrsjdj43-ruby-2.5.7/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:72:in `require'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:273:in `settings'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:84:in `configured_bundle_path'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:351:in `use_system_gems?'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:541:in `configure_gem_path'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:534:in `configure_gem_home_and_path'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:66:in `configure'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:134:in `definition'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler.rb:101:in `setup'
    /nix/store/li2vmpp6mxqzrs6s95szj1xga380hsv4-sample-rails-app-dev-bundler-env/lib/ruby/gems/2.5.0/gems/bundler-1.17.3/lib/bundler/setup.rb:20:in `<top (required)>'
    /nix/store/928i2cf1wizgm7dapmnk0m8ijrsjdj43-ruby-2.5.7/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:72:in `require'
    /nix/store/928i2cf1wizgm7dapmnk0m8ijrsjdj43-ruby-2.5.7/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:72:in `require'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/ruby_supportlib/phusion_passenger/loader_shared_helpers.rb:363:in `activate_gem'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/ruby_supportlib/phusion_passenger/loader_shared_helpers.rb:221:in `block in run_load_path_setup_code'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/ruby_supportlib/phusion_passenger/loader_shared_helpers.rb:527:in `running_bundler'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/ruby_supportlib/phusion_passenger/loader_shared_helpers.rb:220:in `run_load_path_setup_code'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/helper-scripts/rack-preloader.rb:91:in `preload_app'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/helper-scripts/rack-preloader.rb:189:in `block in <module:App>'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/ruby_supportlib/phusion_passenger/loader_shared_helpers.rb:380:in `run_block_and_record_step_progress'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/helper-scripts/rack-preloader.rb:188:in `<module:App>'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/helper-scripts/rack-preloader.rb:30:in `<module:PhusionPassenger>'
    /nix/store/h43g73ynp58fkfphvb8r874sq09ldlpa-passenger-6.0.4/src/helper-scripts/rack-preloader.rb:29:in `<main>'

This fixes the problem: zetavg/nix-packages@aca64f4
Differences between the Bundler Ruby version and the application Ruby version, need to manually give RUBYLIB so that the application Ruby can load Bundler libs from a Bundler that isn't prepared for the application Ruby.