Rerun (restart) stopped instance
It is great that we have a container on the disk which is not runnin any more, but what can we do with it?
Probably the most important thing is that we can re-launch it. Type in docker container start -i ubu and you'll be back inside the same container. How do you know it is the same? You can run htop and see it is installed. If you were running a new container you'd have no htop installed as that's not part of the Ubuntu image as it was downloaded from the Docker hub.
docker container start -i ubu docker container start -i CONTAINER