First let be clear that I am not a Docker expert. Unfortunately there is no Docker expert among the few people regularly replying on this forum so I’ll try to answer you to the best of my capability.
I think that this comes from the fact that Docker is a container technology. That means that a Docker container can’t access by itself any of the environment of the hosting computer. When it concerns files on the hosting computer, or files that can be accessed from the hosting computer, it means that these configurations should be done ‘blind’ because otherwise it would break the container limitations.
What you have to do is to setup the mounts to the directories to backup in your Duplicati container configuration (IIRC it’s done in the docker file). I don’t remember if somehow this is adding the paths automatically to the backup job, if not you should add the paths manually in the source screen (the third tab in the backup configuration).
Suggests your container came from an image from the LinuxServer developers.
Do you know where your container is from? What operating system are you on?
Can it see /config, /backups, and /source? If so, that suggests linuxserver too.
See their Application Setup directions to set those as desired, or use system tools. How to use this image is similar information for the images that Duplicati releases.
You’re issue is that docker needs host volumes mounted to a specific path like /source.
If your host has a file directory such as /home/xtophe, then you need to run your docker command with ‘-v /home/xtophe:/source’ which make your files available from the docker container with the path /source