This is an example application built from a dockerfile. It is a minimal Ruby Sinatra application that return the user's IP address when they visit the site.
The Dockerfile installs Ruby 2.1.2, building it from source.
See https://registry.hub.docker.com/u/craic/docker_sinatra_example_1/ for the same application built from a Dockerfile which uses the packaged Ruby (1.9.3) linked to Ubuntu 14.04
All source code is available at https://github.com/craic/docker_sinatra_examples