How can I use configs which are set in the runtime.exs

I would like to set the environments for my plugin at runtime. Therefore, I would like to use the config/runtime.exs for configuration. Unfortunately, this is not working, as only the content of the _build/prod/lib/rel//lib is packed into the tar file.
Is there a way to use the runtime.exs for configuration, or is there any other way to provide configuration at runtime?

Thanks
Stefan