This simplifies the containerd installation instructions. All Linux distros will
now download from Docker repos, including Ubuntu 18.04 which previously
installed from Ubuntu repos.
The Docker runtime instructions have also been simplified. The RHEL/CentOS
specific option overlay2.override_kernel_check=true option has been removed.
It seems Docker will now autodetect overlay2 support on older Linux kernels
<4.0.0,>=3.10.0-514, which have back-ported overlay2 support on RHEL/CentOS 7.4+
See moby/moby#34368
Terry Moschou
e7e5f0c912