Replying to my own question here.
It seems this is caused by newer versions of Docker, I don’t know which ones exactly, that needs this new package. My guess is that it’s not yet available in GCP apt-get packages, hence the error.
I was previously working with v19.03.15 of Docker which was working fine. Therefore I’ve forked the machine project, added an engine-version flag that I need to set to 19.03.15 so that when executing the script located at https://get.docker.com/ it adds an environment variable VERSION=19.03.15.
You can find the fork here if you ever need the same fix. However I hope the docker-ce-rootless-extras package will soon be available.
Cheers