silence clear-docker-cache

What does this MR do?

This rewrites the clear-docker-cache script to be more silent.

Why was this MR needed?

It is necessary to ensure this can be called from a cron job without polluting mailboxes.

What's the best way to test this MR?

Unsure: I suspect this script is not part of unit tests, but I haven't verified that.

What are the relevant issue numbers?

No issue has been reported yet, Gitlab.com reports a 500 error while searching the issue queue here.