Christian,
I am running HA on a Ubuntu docker container and can discover my Sonos devices without problems:
Could this be a firewall issue which only impacts the docker container running in network=host mode (using same IP as your NAS), but not the VM which likely gets a different IP assigned?
If you have a firewall enabled on your Ubuntu NAS, switch it off temporarily to check if Sonos discovery works without the FW. If yes, switch it back on and open up the necessary ports. There is a post discussing which ports you need to open up for auto-discovery:
Cheers,
Lutz