Add clear-docker-cache script to runner image

In the great image refactor of 18.0, we inadvertently dropped the clear-docker-cache script from the runner images.

Arguably the base images is a better place to put this, however, this script must exist in this repo because it is also included in the runner OS packages, and I do not want to duplicate it in another repo.

This addresses Missing clear-docker-cache script in the Docker... (#38571 - closed)