Add optional authentication

[eznix86]

I think something like this will do
https://smartshitter.com/musings/2017/12/nginx-rtmp-streaming-with-simple-authentication/
or this:
https://smartshitter.com/musings/2018/06/nginx-rtmp-streaming-with-slightly-improved-authentication/

[eznix86]

Like you did in the docker-compose.yml file, you add:

# This enable authentication on the rtmp.
# e.g rtmp://docker-broadcaster.com/stream?access_token=PWTAv&U)NmpzDZ1fm:+uT>r|"B3r!@
# e.g rtmp://docker-broadcaster.com/stream/live?access_token=PWTAv&U)NmpzDZ1fm:+uT>r|"B3r!@
ACCESS_TOKEN: "PWTAv&U)NmpzDZ1fm:+uT>r|"B3r!@"

If the access_token is null, there will be no authentication.