geth not upgradable via apt-get because ethereum apt repository missing by default

[Cory Gabrielsen (cory.eth)]

Issue:

sudo apt-get update
sudo apt-get upgrade

does not update geth.

Root Cause:
The ethereum Apt Repository is not installed

Resolution (suggested):
Run the following commands and re-create the base image

sudo add-apt-repository ppa:ethereum/ethereum
sudo apt-get update
sudo apt-get upgrade