You don't need to use your own container image at all, if you're not familiar with docker and Gitlab CI it's better to skip that step and use the default image. Learn one thing at a time ;-)
The reason gitlab CI/docker explanations are even in there is that it can be tricky to setup a Haskell pipeline "correctly". If you have no caches, builds are going to take a looooong time. But there are better people to learn those technologies from.
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