Always quote anything containing asterisks or the shell might replace it with a matching filename.
Also, the data is stored in /var/discourse and it might not delete any images or containers docker has created or downloaded; those should live in /var/lib/docker, but that’s a blind guess. (They’d not hinder or affect a new installation anyway, just fill up some disk space.)
And does apt-get know that it needs to stop containers and the docker daemon itself before uninstalling the package?