config.ru

require 'rubygems'
require 'bundler/setup'
require 'sinatra'

module Sample
  class ApplicationController < Sinatra::Base
    get '/' do
      return 'Hello World'
    end
  end
end

run Rack::URLMap.new('/' => Sample::ApplicationController)

Dockerfile

FROM gurpartap/ruby-build:2.1.2

RUN mkdir /app
WORKDIR /app
ADD Gemfile /app/Gemfile
RUN bundle install
ADD . /app

EXPOSE 3000
CMD bundle exec thin start

Gemfile

source 'https://rubygems.org'

group :development do
  
end

gem 'rake'
gem 'bundler'

gem 'thin'
gem 'sinatra'