Code is never self-contained. It runs in an environment. Docker lets you define that environment in a simple and portable way. That’s why pretty much every automated testing and deployment service works with docker containers. You give them a container, and done! But how do you set up a container to use for testing your Rails app? In this article, Milap Neupane will show you. https://www.honeybadger.io/blog/testing-rails-with-docker/