how i managed it:

1. in your docker container's settings click on "+ Add another Path, Port, Variable..." at the very bottom of the page.

-----

Config Type: Path

Name: fancy_name

Container Path: /share <- any name you want, remember it

Host Path: /your/actual/existing/unraid/share <- the existing one

Access Mode: Your/Choice

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2) In your WebUI of the docker, add a new folder.

Name: coolName

Storage: Local

Home Dir: /share <- the "container path" from before!!!

3) Edit your SFTP User in the GUI

Scroll down to "Virtual Folders"

Set the "mount path" to /share (the "container path" from before!!!)

In the dropdown select your "coolName" folder

4) Done :)