Docker 18.09 removed the deprecated
AUFS filesystem support and made
overlay2 the default.
AUFS has been the default for a while and therefore most of the updates have been fine. Unfortunately, because
there doesn’t seem to be a clear upgrade path for boot2docker-based VMs, the only suggestion we have is to destroy and
rebuild. This will not delete your project files but it will destroy the Docksal VirtualBox VM and rebuild it.
You can skip the VM update temporarily, create DB backups/etc., then restart the update to complete it.