Are you using Unraid? If yes, just use the template as is and it will work, you don't need to pass through any other variables, everything is preconfigured and nothing needed.
The default configuration will work OOB:
("--key _" ensures that your server requires clients to use the key which is generated on first start)
Please also keep in mind my container is not the one that you've linked since my container contains both the Server and Relay.