GitLab PM here. Auto DevOps makes use of herokuish in order to find the language of your project. Herokuish images are Ubuntu-based, and it doesn't allow the use of specialized images. One can always provide a dockerfile (based on the image of your choice) and Auto DevOps will use that instead.
At least the ones where the generic container is Ubuntu/Debian based. `apt update && apt install -y` is really slow. But alpine based containers with the equivalent `apk install` are blazing fast.