The recent release of ShinyProxy 3.1.0 features a new way of retrieving the username and usergroups using HTTP headers instead of environment variables. This makes it possible to still identify a user when using container pre-initialization and sharing.
We recently created a demo app, explaining how to do this in Shiny: https://github.com/openanalytics/shinyproxy-shiny-demo-auth
The app can also be used to quickly debug which HTTP headers and environment variables are available.