docker-worker: check docker version
As explained in this MR 941acad6 ,
docker-worker doesn't work with old version
18.09.1 docker that installed via
apt install docker.io. We have updated doc to install latest docker from Docker's official repo.
As @terceiro proposed, we should also have a docker version check somewhere. At least in the debian package dependencies, and maybe also in the
docker-worker service before the startup.