How to use Git in a monorepo with multiple dev containers for subfolders?

I have a monorepo structured like this:

```
main-repo/
.git
.devcontainer/
sub_folder_devA/
sub_folder_devB/
sub_folder_devA/ # has its own Docker setup
sub_folder_devB/ # has its own Docker setup
sub_folder_devC/ # shared utility code
```
Each dev container is set up for a specific subfolder (e.g., sub_folder_devA or sub_folder_devB), but both also mount sub_folder_devC as a dependency:
/sub_folder_devA/sub_folder_devC
/sub_folder_devB/sub_folder_devC

I want to use Git inside each dev container, but have it work with the monorepo’s .git directory, so I can see and commit changes for the relevant subfolder.
Despite trying various workspaceMount and workspaceFolder settings, I can’t get Git to work as expected in the dev containers.