Build Docker images in GitLab CI

When implementing GitLab CI to build docker images, there were a few steps that weren’t immediately obvious from the documentation.

First, you need to add Docker as a service.

services:
  - docker:19.03.8-dind

Second, you need to install the docker CLI. For debian-based systems, that’s docker.io, not docker.

before-script:
  - apt-get update
  - apt-get install docker.io -yqq

Third, you need to point the docker CLI to talk to the Docker service.

variables:
  DOCKER_HOST: tcp://docker:2375/