* chore: Disable `smtputf8` support in config
This was always configured disabled at runtime, better to just set explicitly in `main.cf` unless config diverges when Dovecot is enabled to opt-out of this feature.
* remove PostSRSD wrapper
The setup is now completely done during _actual_ setup phase. The
wrapper did not even catch signals (SIGINT, etc.), which I think is
strange.
I also added all the ENVs the wrapper relied on (which previously could
have been unset) to the variables script.
* forgot adjusting the `Dockerfile`