Require Docker CE 18.06

[rickmanelius]

Is your feature request related to a problem? Please describe.

• Documentation update and requirement check.

Describe the solution you'd like

In the current Docker Volumes and Snapshots PR, previous versions of Docker CE have an instability that would make such a feature untenable for those that were not prompted to use version 18.06. Considering that many have been using this for over a week and no major gotchas have come up, I think it's worth pressuring users to upgrade to this version.

Describe alternatives you've considered

I think the alternative of not pushing for an upgrade will have more users hitting upon this instability if/when we release the feature. To that end, I think it's worth the small inconvenience of the upgrade for the benefit of not being frustrated or experience dataloss.

I'm open to be convinced otherwise, but my bias is to push for 18.06.

[rfay]

I've already changed the constraint in #987, we normally don't make specific version statements in our docs because they're too hard to maintain. But we will have to push this in the release notes and announcements.

[rickmanelius]

Fantastic. I had noted this as a follow-up from sprint planning and wasn't sure if was connected into that issue already. Closing this as a duplicate because it's handled there. Thanks!