from you can set the docker directory to another one - but only if that alternate is also on the same filesystem. (begging the question: why even bother?)

1. stop your shit

   $ sudo systemctl stop docker.socket
   $ sudo systemctl stop docker.service

2. edit /lib/systemd/system/docker.service - find ExecStart= and put a -g <dir> before the -H, like so:

   ExecStart=/usr/bin/dockerd -g /new/path/docker -H fd://

3. prepare that directory. make sure it exists, has the permissions you expect (probably want to chown -R :docker on it)
4. reload, restart

   $ sudo systemctl daemon-reload
   $ sudo systemctl start docker

5. test

   ps aux | grep -i docker | grep -v grep

   you should see the -g you added. then you can go into the directory and make sure docker put all its shit there.

from you can recursively mount any linux directory in any other - but docker trips over not being allowed to create hard links, and fails.