Docker system prune

docker system prune will delete ALL dangling data. even unused data, with -a option.

You also have:

• docker container prune (delete all container)
• docker image prune (delete all image)
• docker network prune(delete all network)
• docker volume prune(delete all volume) 

For unused images, use docker image prune -a (for removing dangling and ununsed images).
Warning: 'unused' means "images not referenced by any container": be careful before using -a.

As illustrated in A L's answer, docker system prune --all will remove all unused images not just dangling ones... which can be a bit too much.

cr. https://stackoverflow.com/questions/32723111/how-to-remove-old-and-unused-docker-images