This is a simple demo application using Docker to create and run a Sinatry Ruby application. I have run this on a Mac through boot2docker with ports 49000-49900 forwarded.
First build a ruby docker Image:
$ docker build -t mwarren/ruby ruby
This starts with a base image of dockerfile/ubuntu which can take a while to download. Then it compiles and installs ruby.
$ docker images
The hello image is a Docker image that is somewhat production like. It's using foreman/thin as a server rather than WEBrick.
The Dockerfile installs bundler, copies data into the container and installs required gems.
$ docker build -t mwarren/hello hello
$ docker run -d -p 49090:4567 mwarren/hello
$ open localhost:49090