Serve a static build folder using Rack when running Capybara

env.rb

require 'rack_directory_index.rb'

$documentRoot = File.expand_path(File.join(File.dirname(__FILE__), '..', '..', 'build'))

Capybara.app = Rack::Builder.new do |builder|
  puts "Creating static rack server serving #{$documentRoot}"
  use Rack::DirectoryIndex
  run Rack::Directory.new($documentRoot)
end

Capybara.configure do |config|
  config.run_server = true
end

rack_directory_index.rb

module Rack
  class DirectoryIndex
    def initialize(app)
      @app = app
    end
    def call(env)
      index_path = ::File.join($documentRoot, Rack::Request.new(env).path.split('/'), 'index.html')
      if ::File.exists?(index_path)
        return [200, {"Content-Type" => "text/html"}, [::File.read(index_path)]]
      else
        @app.call(env)
      end
    end
  end
end