1998: Fix healthcheck r=mergify[bot] a=ghostwheel42
## What type of PR?
bug-fix
## What does this PR do?
- fixes health check of roundcube container (introduced with sso / endless redirect)
- silence logging of health checks
- write php error log directly to /dev/stderr
### 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
- [ ] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.
Co-authored-by: Alexander Graf <ghostwheel42@users.noreply.github.com>