The images are built using mkosi. mkosi required version is 5 or later.
If your distro has an older version, you should install it from source. Dependencies are listed on mkosi README. Python 3.6 is needed for mkosi 5.
Then download the repository and run the installation:
git clone [email protected]:systemd/mkosi.git
cd mkosi
sudo python3 setup.py install
Usage is printed with this command:
mkosi --help
The commands used to build the images are in the Makefile.