I was also after some help. My config looks exactly like wmaarten's and is placed in my /mnt/user/appdata/mopidy folder. Under container variables i have my music directory mapped to /media but when i run mopidy localscan from the console (right click, menu option) i get "WARNING local/media_dir must be set."
If i cd around i can see that my music has successfully mounted. So i wondered if the container was reading some other config
I've had a go at install mopidy for myself on a spare raspberry pi before with no luck. But I notice in the container that mopidy.conf also exists in the root directory which i think is for initial setup using a user but the container points to etc/mopidy which is the case for a system service. But mopidyctl local scan doesn't seem to work either. \