Please check the second share path in your docker script. If you are using this one:
there might be a syntax error in the exact line. If /share/Multimedia/Music is the correct folder where your music is on the NAS, the script line should read:
/share/Multimedia/Music:/Music
Apparently the :/Music part is missing, which defines the alias name of the folder inside the container. Roon in the container seems to take the share post on the host side (/share/Multimedia/Music) as the alias name within the docker, which leads to nowhere because the first term of the line, before the : is missing.
Don´t forget to update or reinstall the container after altering the script.