[docker] How can I delete Docker's images?

Possible reason: The reason can be that this image is currently used by a running container. In such case, you can list running containers, stop the relevant container and then remove the image:

docker ps
docker stop <containerid>
docker rm <containerid>
docker rmi <imageid>

If you cannnot find container by docker ps, you can use this to list all already exited containers and remove them.

docker ps -a | grep 60afe4036d97
docker rm <containerid>

Note: Be careful of deleting all exited containers at once in case you use Volume-Only containers. These stay in Exit state, but contains useful data.