I can’t help with the specific issue, but I think it is more Docker specific, so maybe Stack Overflow or something similar has a better audience for it?
What I can offer is to suggest the new feature where you can have CIFS/SMB shares as sources, which was added to 2.1.0.110:
It is a bit clunky because the UI does not support it yet, but if you use the ngax UI, you can manually edit the source paths and type in the new format.
Using this, you do not need the folders to be pre-mounted, as the connection is done directly from Duplicati when the backup runs.