Rebased PR: add docker compose for installing a chatmail relay

[missytake]

Thanks @Keonik1 for #614 :) I had a lot of fun with git rebase to break it up into small, reviewable chunks. It's still a lot, but doable :)

Before we can merge this, we need to merge:

• cmdeploy: cmdeploy run option to skip DNS checks #661
• cmdeploy: prepare for being able to run commands in docker containers #660
  • which depends on cmdeploy: allow to run SSH commands locally #659
• cmdeploy: start and enable fcgiwrap #657
• cmdeploy: add --force to cmdeploy init for recreating chatmail.ini #656

Additional related PRs:

• cmdeploy: add acme_email config value #663 - that one is maybe necessary, but I don't think so
• docker: add traefik support #662

With this PR, checking DNS records should already work; Getting tests to run with a docker instance is still a bit of work (see the docker-rebase...docker-tests branch for first steps).

So for now this will be an experimental feature which we can tell people to try out if they ask for it. But in the long term we need a separate CI machine for testing the docker setup, to be sure nothing breaks when we make changes.