Docker - Unable to locate package docker-engine

The docker package already inside Ubuntu is called docker.io [1] so just do

sudo apt-get install docker.io

But if you follow that link you gave and do steps 7, 8, 9 then your installation will know about the package at the docker repo and also find docker-engine.

Your call. I run the Ubuntu version (currently 0.11.2 on Ubuntu 16.04) on some machines, and the one from Docker on others (as I was curious about some 0.12 features). Both will work just fine.

[1] As docker is used for a desktop launcher application 'docking' icons.