I might have understood what the problem is. 
I thought I was supposed to be teaching Sonarr to talk to rtorrent (not rutorrent) through its XMLRPC capabilities, thus as I was saying using either the socket file (which is the default config, also, I can’t export it as a volume -> How to export rtorrent socket from container) or as a classic localhost:5000 host/port combo.
instead, your default vanilla configuration is teaching Sonarr to talk to rutorrent through the same localhost:80 host/port combo that the user would type in to get to the WebUI!
in any case, I still cannot get it to work so I guess my docker-compose is wrong.
does my network configuration seem reasonable to you?