1082: Use socrate instead of Mailustart r=mergify[bot] a=hoellen
## What type of PR?
enhancement
## What does this PR do?
This PR updates the `Dockerfile`, `setup.py` and `config.py` of each image to support the new [Mailu/socrate](https://github.com/Mailu/socrate) python package. So [MailuStart](https://github.com/Mailu/MailuStart) is not used anymore for resolving DNS and configuration processing.
## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.
- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.
Co-authored-by: hoellen <dev@hoellen.eu>