This seems to be the best working example of running a rails app in docker. This is just an empty rails app but it compiles with the following commands.
docker-compose build docker-compose up
Files of note: .env.* Dockerfile docker-compose.yml
#Links This is the tut I ran through http://blog.carbonfive.com/2015/03/17/docker-rails-docker-compose-together-in-your-development-workflow/