If Docker is not installed, explain how to install Docker

[bfirsh]

Cog depends on Docker to package models. The installation instructions in our docs specify that installing Docker is a prerequisite to using Cog, but it is still possible to install Cog without installing Docker. Let's update the cog CLI to check whether Docker is installed, and exit with a helpful error message if Docker is not installed.

edited by @zeke

[mcam10]

Is this still needed? @zeke @bfirsh