3008: Fix issues with log filter and remove POSTFIX_LOG_FILE r=mergify[bot] a=Diman0
## What type of PR?
bug-fix
## What does this PR do?
Fixed log filter not filtering out log messages for dovecot/nginx/postfix.
Fixed postfix not logging to standard out.
Fixed not all containers logging to journald.
Removed POSTFIX_LOG_FILE functionality. A new FAQ entry is created that documents how to log to file with journald & rsyslog.
Thank you `@Lex999` for providing the sample code in #2839 for how to capture the standard out of called sub processes.
### Related issue(s)
- closes#2839
- closes#2819
- closes#2939
## Prerequisites
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/workflow.html#changelog) entry file.
Co-authored-by: Dimitri Huisman <diman@huisman.xyz>
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