@m2-farzan Nice find, but this PKGBUILD script just uses colcon as well, nothing special here...
Also, I have just emailed the ros2-foxy-desktop package maintainer, if he could provide the source, so thanks for that.
The CI infrastructure comment you mention is basicly what I am trying to do on a local level, but it is not supported for Debian builds. In short, it's a real pain to set up all the different config files, which are not fully documented.