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Before this, if registry or tag got set, we would still build all images
starting from the docker hub regsitry version and latest tag.

This makes it easier for images that build on ours to use tools like packer
without having to specify the full path

Add a cleanup script for ns-3 similar to those for qemu and gem5.

This solves #9.

Removes source and objects for qemu and linux kernel, packer cache, and
raw images.