The issue I found with this container re: ssh is that there is no home folder setup for the app user.
So when creating ssh keys they get written to /dev/null/.ssh by default (go into the ether). This is also the location that known hosts should be, so even if you put ssh keys somewhere else when vorta/borg attempts to ssh to borgbase it can't save the remote host into the known hosts file.
To rectify I jumped into the Vorta2 container:
docker exec -it Vorta2 sh
Then setup /config as the home directory for the app user (that way keys and known hosts would live in my appdata/Vorta2/.ssh folder)
/tmp # usermod -d /config app
To check it worked you can see where users home folders are located by running this in the container:
getent passwd
...
app:x:99:100::/config:/sbin/nologin
...
From there I could create ssh keys, select them from dropdown, and connect to borgbase with authorized hosts and keys being stored in /config/.ssh
My vorta appdata folder now has the following in its .ssh folder:
Hope that helps.