How to access external USB drive as destination, hooked up to NAS (duplicati in docker)

While I’m stuck setting up a backup task from one NAS to another (see here) I thought I’d try a USB-drive as backup destination, hooked to the source-NAS (Synology DS220+). On the NAS I installed Duplicati in a docker-container. Container runs OK and I can access Duplicati’s config page via http://MyNas1:8200/ngax/index.html.
In Step2 however (setting up destination), I am stuck: After selecting “Local folder or drive” as storage type I don’t find my drive in the browser below. The browser displays the usual Linux-like system folders on my NAS, but nothing after /var. On the DS220+ the drive appears to be correctly mounted and is accessible in Filemananger. I also find it (with Putty) in the filesystem under /volumeUSB1/usbshare. But I have no clue what I need to do to get this usbshare to appear in duplicati’s browser.
I created a folder //volumeUSB1/usbshare/Duplicati with Putty, but when I directly enter this path in duplicati and ‘try connection’ it claims that this doesn’t exist. When I confirm to create it I get access denied.

Any clues how I get to my USB-drive, anyone?

EDIT: Sam question if I want to backup local data to any destination with duplicati. How do I specify source files under /volume1/…?

it’s obvious that it’s a Docker problem, I am sorry to not be able to help you further than to point you to the existing resources, for example the forum. From this link, I’m not sure that there is documentation about Docker in the Duplicati doc itself. I have not found any but I’m far from having read it exhaustively.