Add a Docker example for running Riak

This pull request adds a Riak service example to the Docker documentation website.

Additional details on how you'd go about deploying Riak across multiple containers that can talk to each other is covered in the [docker-riak](https://github.com/hectcastro/docker-riak) project.

Also, I'm relatively new to Docker, so and advice or feedback to make this example simpler is welcome.