Unpin docker and move to latest version

[jodh-intel]

#374 pinned docker to the last known good version of docker that supported device-mapper. However, as shown in #373 (comment), docker has released a number of new versions.

Undo the pinning and switch strategy to simply make a note to users that if they wish to use a feature such as device-mapper, they will need to look at alternative configurations.

[ganeshmaharaj]

The two alternate device mapper / lvm solutions for containerd are.

1. https://github.com/firecracker-microvm/firecracker-containerd/tree/master/snapshotter/cmd/devmapper
2. https://github.com/ganeshmaharaj/lvm-snapshotter

Both of them are for containerd. devicemapper will currently pending merging into containerd upstream repo via containerd/containerd#3022.
crio supports a similar LVM /devicemapper snapshotter.

It would be nice to tell the user that if they wish to use a block backend for docker they can run the stand-alone versions of the snapshotter until the backend lands after which they can use devicemapper / lvm as their backends.

[jodh-intel]

Thanks @ganeshmaharaj - I've added a note to all the docker install guides on #409 pointing back at this issue so users can find details on alternatives.

[ganeshmaharaj]

A latest update from docker engineering team. They are currently working to make docker use containerd to as the default storage handler. Refer to comment docker/for-linux#452 (comment). Hopefully the two changes referenced there will land soon. We will keep an eye on those and update once they land in mainstream docker.

[zeigerpuppy]

looks like it's fixed in docker-ce 19.03.1 and upwards. I haven't tested yet on my Devuan box...

docker/for-linux#452 (comment)