Simplify deployments/upgrades

Ideas etc. for simplifying the deployment/upgrade process:

* cut git out of the install?
* pre-building all images?
- this is what we already do for the SMTP server image, although build process is currently manual
- make builds reproducible
- make installs/upgrades faster(?)
- less disk space required on host machines(?)
- use github container registry to serve images?
- may have to allow more configuration to be done via env var, e.g.
- changing database
- disabling sentry
- different SSL termination options(?)