my storage does not using

[a-x-]

docker puts files into /var/lib/docker/overlay2/1c5c2a0de7a49e46ec984252a8c092b503eb366f1f176b9c35f352f4e227b5c5/merged/mnt/timemachine

instead of /mnt/timemachine

do you know, in what direction I can search to fix it?

[a-x-]

I used incorrect (external) add-account VOL_ROOT param (/mnt/timemachine).
I fixed it when I had changed VOL_ROOT to /timemachine