Note: This currently doesn’t work due to libc issues on most Ubuntu versions. I am working on a solution !
I use fcct, the Fedora CoreOS Configuration Transpiler in order to create Ignition JSON files for installing CoreOS from YAML
Use these commands in your shell to download fcct to /usr/local/bin/fcct and hence install it. This works on most Linux distributions, but I mainly use it in Ubuntu and Debian.
sudo wget -O /usr/local/bin/fcct https://github.com/coreos/fcct/releases/download/v0.9.0/fcct-x86_64-unknown-linux-gnu chmod a+x /usr/local/bin/fcct
See Fedora CoreOS minimal ignition config for XCP-NG for my Fedora CoreOS ignition config