The following is a radical solution. IT DELETES ALL YOUR DOCKER STUFF. INCLUDING VOLUMES.
$ sudo su
# service docker stop
# cd /var/lib/docker
# rm -rf *
# service docker start
See https://github.com/moby/moby/issues/22207#issuecomment-295754078 for details
It might not be /var/lib/docker
The docker location might be different in your case. You can use a disk usage analyzer (such as mate-disk-usage-analyzer) to find the folders which need most space.
See Where are Docker images stored on the host machine?