Issue with VirtualBox missing from Docker-Toolbox install

[bfg111]

OSX El Capitan
Version 10.11.1

Getting the following error when launching in terminal:
'$ bash --login '/Applications/Docker/Docker Quickstart Terminal.app/Contents/Resources/Scripts/start.sh'
Either VirtualBox or Docker Machine are not installed. Please re-run the Toolbox Installer and try again.'

[jmorganca]

This means the user already had VirtualBox installed, but I'm not sure why it's not being picked up by the Docker tooling.

It's possible the user may have had an older or corrupt version of VirtualBox. Maybe a binary is missing.

I'm looking into a fix for our detection, but for now the best solution is to ask the user to install virtualbox manually from https://www.virtualbox.org/.