Enable features via environment variables

[thschue]

Hello Everyone!

I'm currently trying to automatically configure icinga with this Dockerfile. Many things are working pretty good, but I'm currently dealing with the problem that I cannot enable features (in my case influxdb) automatically (instead of icinga2 feature enable influxdb). I found the PR here: https://github.com/Icinga/docker-icinga2/pull/18 but this doesn't seem to be very active anymore. Is there already another way to achieve this or is this not possible at the moment? For icingaweb2 this worked pretty smooth for modules and env-variables.

Thank you in advance!