The Rails application in this container does nothing. It's a basic Rails app pointing to a PostgreSQL database and runs in development mode (i.e. RAILS_ENV=development). This container is simply an attempt at understanding linkages between containers.
The container should be run pointing to a container named "db" with PostgreSQL running. There are several environment variables expected which can be seen in the config/database.yml file in the GithHub project.
If you run the dharmamike/pmx-pgsql container, you can run this container alongside it simply by running the following command
docker run -t -i -p 80:3000 -link pgsql:db -name railsapp dharmamike/pmx-rails
Then, on the host, you can access the default Rails welcome page with
curl -s localhost