GitLab

Using Git submodules with GitLab CI

Set relative submodule urls:

[submodule "project"]
  path = project
  url = ../../group/project.git

Update submodules before the build:

before_script:
  - git submodule sync --recursive
  - git submodule update --init --recursive

https://docs.gitlab.com/ce/ci/git_submodules.html