Add Docker environment variables to documentation

[jm20122012]

If there are any environment variables, please add the list of available environment variables for the Docker container somewhere in documentation. It may be out there somewhere, but I don't see it on the Docker hub site (https://hub.docker.com/_/nats) or in the NATS documentation here (https://docs.nats.io/running-a-nats-service/nats_docker).

[kunalvarpe]

I think this need some attention