Do not recommend relative URLs for submodules

https://docs.gitlab.com/ce/ci/git_submodules.html#configuring-the-gitmodules-file

Relative URLs for submodules will cause problems once someone forks a project. It's better to always use absolute HTTPS-URLs. To achieve the goal as stated in the documenation (HTTPS for CI jobs and SSH for local checkouts), better use the URL rewriting of Git.

Example how to use SSH instead of HTTPS for gitlab.com:

git config --global url."git@gitlab.com:".insteadOf https://gitlab.com/