All of these assume you're running as root. If not, preface them with sudo
as needed.
docker images --filter 'dangling=true'
To remove them:
docker images -q --filter 'dangling=true' | xargs docker rmi
docker ps -a -f status=exited
To remove them:
docker ps -a -q -f status=exited | xargs docker rm
Cleaning up the dangling images might be useful - see above.