Ubuntu as an Apple Time Machine capsule with Netatalk 3

In 2016 I wrote "Apple's TimeMachine now works with Samba SMB2. Apple also is moving to depreciate AFP which was the previously underpinned TimeMachine" Fast forward to 2020 - what a year:

• I have finally upgraded from Ubuntu 16.04 LTS > 20.04 LTS
• Samba was never really hugely reliable for TimeMachine in my experience
• Netatalk is still around now at version 3
• MacOS I'm running is High Sierra (no Catalina yet) and still has AFP

Tl;DR here is how to set up Time Machine capsule on Ubuntu upgrading to Netatalk 3, from Netatalk 2.