Add option to specify a sleep between each image removed

[mingfang]

When deleting images, Docker appears to consume all the CPU, causing the entire machine to be unresponsive. One workaround would be to add a sleep option to "yield" after each image removed.
One the cli, I would run something like this
docker images -q --filter dangling=true | xargs -I % sh -c '{ docker rmi %; sleep 3; }'

[stale]

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