Update 28 July 2019: An updated version of this guide for Ubuntu Server 18.04 LTS is now available. Feel free to check it out.
Update 23 May 2020: This guide is ALREADY OUTDATED and might no longer work with new versions of Ubuntu and VirtualBox. Please consider switching to the updated guide instead. I will no longer respond to the replies to this gist. Thank you.
This guide will walk you through steps on how to setup a VirtualBox shared folder inside your Ubuntu Server guest. Tested on Ubuntu Server 16.04.3 LTS (Xenial Xerus)