config.ru

use Rack::Static,
  :urls => ["/images", "/js", "/css"],
  :root => "public"

run lambda { |env|
  [
    200,
    {
      'Content-Type'  => 'text/html',
      'Cache-Control' => 'public, max-age=86400'
    },
    File.open('public/index.html', File::RDONLY)
  ]
}

dir structure

- application-name
  |- config.ru
  |- Gemfile
  |- Rakefile
  |- public
    |- index.html
    |- images
    |- js
    |- css

Gemfile

source :rubygems
gem 'rack'

gem 'rake'
gem 'thin', group: :production

Rakefile

namespace :db do
  task :migrate
  task :setup
end