Docker Compose with specific user

I’m running docker in a privileged LXC with a volume mounted to a network-share. The mount is accessible docker (as root), but just visible in my container, which is not what I want.
Therefore I tried to bring the container up as user root (I know, that is not the way it should be done):

user: ${UID:-0}
- /mnt/calibre/Daten/Calibre:/books

But when I inspect the process which is run inside the container, it is still running as user abc (UID: 1000). I’d like to give user abc the permission on volume books, but this user doesn’t exist on the host (LXC).
Any help appreciated!

Just for my understanding: you are running a Docker container inside a LXC?

Yes, there are nice already wrapped packages you can use with docker but not with LXC…