Add environment for docker-compose

[CoMPaTech]

Great work ... ran your container happily for a few hours yesterday ... from the looks of the server-logs it's still running (left it running without players). Hopefully this small addition is useful for someone!

[wolveix]

Hey! Thanks for taking the time to submit this pull request :)

While I agree with adding the environment variable into the Docker examples (even if it's set to false by default), I don't believe the .env contribution really adds anything unfortunately. You can set the value very easily without it, and I feel like it adds another layer of complexity for less experienced users.

I'd certainly appreciate your feedback if you feel differently though!

[CoMPaTech]

Well, you could also suggest someone adding it to the compose-file with true for that matter, this is better way of working but you will need to expose it obviously. Otherwise feel free to close it :)

        environment:
             - STEAMBETA=true

[wolveix]

It's a better way of working for people that understand how to use it. A lot of people that use this image aren't too familiar with Docker or Linux, so I don't see the benefit in adding something that complicates a process that is already incredibly simple :)

Thank you for your contribution, though!

[CoMPaTech]

LGTM!