HAProxy config for Radicale (= CalDAV server)

I’m running HAProxy on a LXC with Debian 8. This is working w/o issues.
Now I want to enhance the functionality to work with Radicale, a CalDAV server in LXC (with Alpine Linux).
Radicale share information to configure a reverse proxy with Nginx.

Can you give advise to translate this into a working HAProxy configuration?