

It’s an immutable distro, so the regular locations on the filesystem are all read-only. The only way you can do this is making sure you mount separate storage, or use the userspace home mount or whatever they call it. It’s trivial to move the docker dir to wherever, so just do that.
No, it’s the difference between your docker service knowing it’s datadir isn’t in /var/lib and not.
Shut down docker. Create a datadir in your writeable mount, change the docker configs to point to the new location, and restart.
Here’s an example: https://linuxconfig.org/how-to-move-docker-s-default-var-lib-docker-to-another-directory-on-ubuntu-debian-linux