How to find which variables are available for use in application.yaml

In a previous post, I’ve seen that one can refer to the user in the YAML file using #{proxy.userId}. The example given was:
container-volumes: [ "/home/#{proxy.userId}:/home/#{proxy.userId}" ]
I’ve seen several posts making references to the Spring Expression Language in this context but this hasn’t been helpful for finding usable variables.
My question is: how do I find which variables, similar to proxy.userId, are available for use in the application.yaml file?
Along the same line, is there a list of environment variables set by ShinyProxy such as SHINYPROXY_USERNAME?