2221: Add support for custom NGINX config r=mergify[bot] a=easybe
## What type of PR?
enhancement
## What does this PR do?
Add support for custom NGINX config. Including *.conf files in /etc/nginx/conf.d same as the default NGINX configuration gives the user more flexibility.
### Related issue(s)
## 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.
- [ ] 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: Ezra Buehler <ezra@easyb.ch>
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