I'm having this same issue with my Docker storage path. Even if I remove docker ("Remove" button in the UI), and have a custom path like /srv/dev-disk-by-uuid-5345931d-d488-4965-a602-ad71746a9644/docker, it still uses /var/lib/docker after clicking Install... Also, there's no /etc/docker/daemon.json file, the directory doesn't even exist.
EDIT: I manually created /etc/docker directory, and daemon.json file. And manually specified the "data-root" option. It works fine now... Why wasn't this created automatically?