How to configure mountpoint for snapshots in docker / portainer

Hi everyone. Recently I set up Kopia on several devices, including my PC and NAS. It works great on the PC and NAS but I have problems configuring the feature for locally mounting snapshots in the NAS.
The NAS instance runs inside a docker container that I configured via portainer.
The example file for docker compose states to configure:

Mount path for browsing mounted snaphots

  • /path/to/tmp/dir:/tmp:shared

e.g. - /mnt/kopia:/tmp:shared

However, whenever I configure a path like that I see a message like
“Error response from daemon: path /mnt/kopia is mounted on /mnt but it is not a shared mount”

How do I configure the path on the host so that Kopia can mount its snapshots there? I understand that I have to create a mounpoint on the host and that it has have the “:shared” property. But a mountpoint as far as I understand has a source and a target. What shall I specify as the source here? I thought that the source basically is the snapshot and that it would be mounted to the folder that I specified, e.g. “/mnt/kopia”.

Can anyone help me with this?

kopia is single standalone binary - docker is overkill here unless you have to deploy it to fleet of computers. Or if you like to tinker with it.

Do not use docker if you struggle with configuring it…