By the time, old images and containers are still on the system and use a lot of disk space.
Show old containers and images
See the docker images with their ID:
docker images -a
See the stopped docker containers with their ID:
docker ps -a | grep Exit or docker ps -a -f status=exited
Remove single containers and images
Remove a container:
docker rm CONTAINERID
Remove an image:
docker rmi CONTAINERID
Force to remove an image if it is still referenced:
docker rmi -f CONTAINERID
A list of additional command options and other useful commands can be found here: https://www.digitalocean.com/community/tutorials/how-to-remove-docker-images-containers-and-volumes