Logstash as a service, how to supply flags?

Hi all,
I haven't been working long with logstash but I can't seem to find a solution for the following problem.
I've set up a docker with log stash (version 2.3), where it runs as a service
Now I am trying to read an environment variable from linux into my logstash.conf using add_metadata_from_env from the environment plugin.

However for this plugin to accept external environment variables, I've got to set the flag "--allow-env".
Now my question, where do I set this ? Can I just supply this parameter as I start the logstash service (e.g.: service logstash start --allow-env) ?

Add it to /etc/defaults/logstash, for debian based OSs, or /etc/sysconfig/logstash, for RHEL based OSs