`Failed to restore file Could not find file` When Testing Backups

It’s clear that it’s a permission problem indeed.
As I said I have no clue about Docker, from a quick browsing of the forum’s archives, there is an official Docker image and if you don’t use it, well, you are on your own to make it work :-). From another post:

I gather that other people are using uid=0 instead of 1000, so it could be worth a try.