Trying to start the docker service after some upgrades fails with the following message:
Failed to start docker.service: Unit is masked.
It turns out that after upgrading or more specifically removing and then upgrading the docker installation in ubuntu (in this particular case in raspberry 4 with Ubuntu 20.04 installed), results in this error.
A search brings up the following:
and from that the following bug post:
So the solution is to run the following to be able to start the docker service (described in the first link above):
sudo systemctl unmask docker sudo systemctl start docker