Minimal one-page rails 5 application, run with `rackup -p 3030`

application.rb

require "action_controller/railtie"

class TestApp < Rails::Application
  secrets.secret_token    = "secret_token"
  secrets.secret_key_base = "secret_key_base"

  config.logger = Logger.new($stdout)
  Rails.logger = config.logger

  routes.draw do
    root "home#index"
  end
end

class HomeController < ActionController::Base
  def index
    render inline: 'Hello, World!'
  end
end

config.ru

require './application'
run TestApp

Gemfile

source 'https://rubygems.org'

gem 'rails', '5.1.3'