Some features of GitLab require a "runner" (in the sense of a "gopher" or a "minion"). A runner "registers" itself with a GitLab instance, and is given tasks to run. Tasks include running Continuous Integration (CI) builds, and building container images.
This is a companion discussion topic for the original entry at https://geek-cookbook.funkypenguin.co.nz/recipies/gitlab-runner/
Rather than let Docker assign IP ranges to my networks, I choose my own ranges, to avoid any possibilty of a conflict. You can see a list of all the ranges I use, at