A nix environment running Docker
Include the gem in your Gemfile
gem 'docker-rspec', '>= 0', { :git => "https://github.com/cyberious/docker-rspec.git"}
Add the following like to the top of your Rakefile
require 'docker-rspec/tasks/docker-rspec'
Once you have updated the two files perform a bundle update and ensure Docker is running. Then you can run bundle exec rake dspec