This isn’t as easy as you think…a normal du -h doesn’t work on /var/lib/docker. This is because of the aufs filesystem docker uses that du by default skips. The proper way to figure out how much space is actually being used involves a few more arguments:

docker -shx /var/lib/docker
Mario Loria is a builder of diverse infrastructure with modern workloads on both bare-metal and cloud platforms. He's traversed roles in system administration, network engineering, and DevOps. You can learn more about him here.