Enable Docker Swarm mode

For truly highly-available services with Docker containers, we need an orchestration system. Docker Swarm (as defined at 1.13) is the simplest way to achieve redundancy, such that a single docker host could be turned off, and none of our services will be interrupted.

This is a companion discussion topic for the original entry at https://geek-cookbook.funkypenguin.co.nz/docker-swarm/docker-swarm-mode/