Test all the things in GitLab CI with Docker by example

What this post added

This post introduces the concept of using Docker within GitLab CI/CD to automate testing for various application types. It provides example `.gitlab-ci.yml` configurations for Node.js, Django, and Unity3D projects, demonstrating how to leverage Docker images for consistent build and test environments. It also includes useful links to GitLab CI/CD documentation, linters, and example projects, as well as guidance on publishing Docker images to Docker Hub from GitLab CI.