The long missing command to clean up all docker images that are not used by any container.
This command is available as docker image bobrik/image-cleaner:
docker run --rm -v /var/run/docker.sock:/var/run/docker.sock bobrik/image-cleaner
Add -dry-run to the end if you want to see what is going to be deleted.
If you want to keep some images, use -exclude image:tag[,image:tag] flag.
Docker client env variables are picked up automatically, so you can access any docker daemon you can access from docker client, just make sure to set the right env variables to make it work.