Traditionally, code in technical books uses GitHub for a good reason: everyone knows it, and it's free for open source (or just public) repositories. We are going to use GitLab though. First, it's free for both public and private projects. Second, it has some features that GitHub lacks, and we will need them for this chapter: more on this later.
This means that before proceeding further, you will need to get yourself an account at https://about.gitlab.com/ (you can use your GitHub account to log in to GitLab with just few clicks).