Install Docker on Ubuntu 18.04

I was running Ubuntu 18.04 daily build on my server just to get ready for upcoming Ubuntu 18.04 LTS release.

Since Ubuntu 18.04 is not yet released, do not provide a away to install docker on Ubuntu 18.04.

To install Docker on Ubuntu 18.04, run

On 2018-02-28, repository have docker version 17.03-2-ce