Submodules in gitlab-runner exec
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### Problem to solve
The page https://docs.gitlab.com/runner/commands/#limitations-of-gitlab-runner-exec does not contain information with regards to setting up `gitlab-runner exec` when you use **Relative URLs** (see: https://docs.gitlab.com/ee/ci/git_submodules.html#configuring-the-gitmodules-file).
### Further details
It would be useful if there was a section explaining how to clone submodules locally with an example so that `gitlab-runner exec` does not fail with an error that a certain repository cannot be found.
### Proposal
Example `.gitmodules` file:
```
[submodule "subdir/repo"]
path = subdir/repo
url = ../subdir/repo.git
[submodule "library"]
path = library
url = ../library.git
```
Assume the repository I'm running `gitlab-runner exec` in is `group/application.git` you run the following clone commands to get it to work:
```
git clone --bare https://gitlab.local/group/subdir/repo.git subdir/repo.git
git clone --bare https://gitlab.local/group/library.git library.git
git clone https://gitlab.local/group/application.git
cd application
gitlab-runner exec shell build
```
issue