docker images are kept in /var/lib/docker

docker images are kept in /var/lib/docker

# du -s /var/lib/docker/*
20    /var/lib/docker/builder
220   /var/lib/docker/containerd
884   /var/lib/docker/containers
30028 /var/lib/docker/image
84    /var/lib/docker/network
5876620 /var/lib/docker/overlay2
20    /var/lib/docker/plugins
4     /var/lib/docker/runtimes
4     /var/lib/docker/swarm
4     /var/lib/docker/tmp
4     /var/lib/docker/trust
188   /var/lib/docker/volumes

you can save a lot of space by deleting any unused images and containers

# du -s /var/lib/docker/*
20  /var/lib/docker/builder
220 /var/lib/docker/containerd
4   /var/lib/docker/containers
2888    /var/lib/docker/image
84  /var/lib/docker/network
163380  /var/lib/docker/overlay2
20  /var/lib/docker/plugins
4   /var/lib/docker/runtimes
4   /var/lib/docker/swarm
4   /var/lib/docker/tmp
4   /var/lib/docker/trust
188 /var/lib/docker/volumes