Decide on docker version strategy

[jodh-intel]

Docker removed devicemapper in 18.09 which complicates life for many (see kata-containers/runtime#1112).

Our install docs setup the docker repo and then help users install the latest version of docker. Given the above, that's a potential problem so we need to find a non-gross way of updating the install docs to install docker 18.06.

The approach needs to:

• be simple.
• work with kata-manager.

Since the docker package will get auto-upgraded from 18.06 -> 18.09 when the user upgrades their installed packages, we're basically going to have to apt-mark hold docker I think:

• Moving to Docker 18.09 does not delete the VM and proxy when the container terminates runtime#926 (comment)

We'll also have to find the appropriate distro rune for each other install doc and then remove it if/when docker can be upgraded once again. And add a "note" explaining what the extra step is for.

/cc @egernst.

[jodh-intel]

The most annoying bit is going to be having to specify the precise package version of docker the user needs to both install and hold given that, for example, on Ubuntu the user will need to run:

$ sudo apt -y install docker-ce='18.06.1~ce~3-0~ubuntu'
$ sudo apt-mark hold docker-ce

That package version format may be different for each distro repo.

[jodh-intel]

Related:

• no devicemapper support in 18.09.x on ubuntu and debian docker/for-linux#452
• Deprecate "devicemapper" storage driver. docker/cli#1424

[egernst]

@jodh-intel -- we have this covered now, right?