1. release-in-a-container - Like 'make release' but in a container.
2. cross-in-a-container - Like 'make cross' but in a container.
3. package-tarballs - (NEW) To package tarballs with a docker dependency

'release-in-a-container' is currently only for testing and is not slated to
be used, but may be useful for testing in some scenarios.

'cross-in-a-container' is meant to be run from the kube-cross image alone.
'package-tarballs' is a companion target that runs from a docker image
to package up the tarballs and images (from cross-in-a-container) for a release.