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

This solves #9.

When we switched to a newer kernel version in c64d3d0e, the
cleanup_images.sh script no longer removed the linux kernel source
folder, since it still used the old version number. Now, I use a regex
to avoid this issue in the future.

This will push the images built by docker-images and docker-images-debug
target, if they exist.

This will result in make docker-images directly generating images with
appropriate tags.

simbricks/simbricks additionally with gem5.opt installed in addition to
gem5.fast. About 1.1GB larger.

(this image takes very long to build)

increases connection limits, disables host fingerprint checking, enables persistent connections