https://about.gitlab.com/pricing/faq-efficient-free-tier/#ma... provides a list of what counts towards the storage limits, and also ways to manage, reduce and cleanup storage.
> Does it cover the size of a git repo? Build artifacts, both temporary and exported to packages? And what about Docker images?
Yes, they all are storage types that add to the overall storage consumption.
> Docker images concern me the most. It's terribly easy to build and use Docker images that are >100MB, and 50 of those easily go beyond 5GB. Two projects with nightly builds easily go beyond that in a month even when doing nothing at all.
You can define container cleanup policies within the registry, matching on age, tags, etc. https://docs.gitlab.com/ee/user/packages/container_registry/...