I think you’re asking how to find what values you should be setting those variables to. This is mostly explained in the comments around the variables in the file itself and besides the API key, probably you just need to replace discourse.example.com with your own forum’s domain name.
Here is the sample file in GitHub in case that makes it easier to read everything: discourse_docker/samples/mail-receiver.yml at main · discourse/discourse_docker · GitHub
The API key comment directs you to the API section of your forum’s admin interface but it doesn’t talk about what kind should be created. Assuming you haven’t renamed your system user, it should look like this when creating the key: (enter your own description)
Then below that, find and tick receive emails:
After saving that, you will be given the API key needed for DISCOURSE_API_KEY.