App Platform does not support relative submodules for github

I’m trying to create an app using a private github repository that contains several submodules that are also private. Because they are part of the same project, I’m using relative addressing (…/submodule.git instead of https://github.com/Mindesk/submodule.git). However, while building the application I stumbled upon the following error:

=> Cloning submodules
   warning: error cloning submodules: repository not found

For now, I’ve hardcoded the full “https://github.com/Mindesk/” prefix, but I’d rather use the relative address, i.e. “…/”.

Is this a known issue? And is there something I can do in order to support relative addressing?