How to prepare for Supercomputer.io (with gotchas)

Parallella owners around the world will connect their boards via the Internet to create the most efficient supercomputer on the planet. Resin.io has built a special micro-SD-card image (using Docker) so adding your Parallella to the mix is relatively quick.
Detailed steps, along with a few gotchas:
Detailed steps, along with a few gotchas: