Nothing *docker core* should ever wind up on the flash drive. (The exception is the templates within /config/plugins/dockerMan, but that's Unraid specific, not docker per se)
But, the storage driver that docker requires for utilizing the "image" as a directory / folder on a ZFS device is not within Unraid. Should be no issues what so ever with having it as a BTRFS / XFS image on a ZFS device. Check Settings - Docker (with the service stopped) to confirm.