(original answer see below)

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Update Sept. 2016: Docker 1.13: PR 26108 and commit 86de7c0 introduce a few new commands to help facilitate visualizing how much space the docker daemon data is taking on disk and allowing for easily cleaning up “unneeded” excess.

docker system prune will delete ALL dangling data (i.e. In order: containers stopped, volumes without containers and images with no containers). Even unused data, with -a option.

You also have:

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.

Combining docker xxx prune with the --filter option can be a great way to limit the pruning (docker SDK API 1.28 minimum, so docker 17.04+)

The currently supported filters are:

• until (<timestamp>) – only remove containers, images, and networks created before given timestamp
• label (label=<key>, label=<key>=<value>, label!=<key>, or label!=<key>=<value>) – only remove containers, images, networks, and volumes with (or without, in case label!=... is used) the specified labels.

See “Prune images” for an example.

I usually do:

docker rmi $(docker images --filter "dangling=true" -q --no-trunc)

I have an [alias for removing those dangling images: drmi]13

The dangling=true filter finds unused images

That way, any intermediate image no longer referenced by a labelled image is removed.

I do the same first for exited processes (containers)

alias drmae="docker rm $(docker ps -qa --no-trunc --filter "status=exited")"

As haridsv points out in the comments:

Technically, you should first clean up containers before cleaning up images, as this will catch more dangling images and less errors.

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Jess Frazelle (jfrazelle) has the bashrc function:

dcleanup(){
    docker rm -v $(docker ps --filter status=exited -q 2>/dev/null) 2>/dev/null
    docker rmi $(docker images --filter dangling=true -q 2>/dev/null) 2>/dev/null
}

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To remove old images, and not just “unreferenced-dangling” images, you can consider docker-gc:

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A simple Docker container and image garbage collection script.

• Containers that exited more than an hour ago are removed.
• Images that don’t belong to any remaining container after that are removed.