Ask HN: How can I handle my Gitlab CI process for our WordPress plugin?

As a junior devops engineer, I have been trying to create a docker container for our wordpress and mysql so we would be able to use both on gitlab CI for testing purposes. However, the volume and image makes around 9gb and I am not allowed to put them into our repository either. I was thinking of downloading them from an external source every time we run the pipeline but this just feels wrong. How can I handle gitlab CI pipeline process correctly? I fell like I am doing something very wrong.