images: Adds --purge to docker manifest push command
images: Adds --purge to docker manifest push command

The --purge argument will cause the manifest list to be deleted after it was pushed, allowing a new, fresh, and up to date manifest list to be created next time the BuildImageManifestLists.ps1 is called.

Fixes:  #40
kubernetes-sigs/windows-testing,kubernetes-sigs/windows-testing