Ruby Sinatra docker app for (not just) Kitematic

app.rb

require 'sinatra'

class HelloWorldApp < Sinatra::Base
  get '/' do
    "Hello, world!"
  end
end

config.ru

require './app'

run HelloWorldApp

Dockerfile

FROM ubuntu:14.04

RUN apt-get update -qq && \
    apt-get install -y make curl -qq && \
    apt-get clean && \
    curl -sSL "https://github.com/postmodern/ruby-install/archive/master.tar.gz" -o /tmp/ruby-install-master.tar.gz && \
    cd /tmp && tar -zxvf ruby-install-master.tar.gz && \
    cd /tmp/ruby-install-master && make install && \
    apt-get update && \
    echo "gem: --no-rdoc --no-ri" >> ~/.gemrc && \
    ruby-install -i /usr/local/ ruby -- --disable-install-rdoc --disable-install-ri && \
    gem update --system && \
    gem install bundler

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

EXPOSE 3000
CMD bundle exec rackup -p 3000
# for rails use:
# CMD bundle exec rails server -p 3000

Gemfile

source 'https://rubygems.org'

gem 'sinatra'

great !
thanks :)