Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Create new targets for running in existing containers (GCB).
Create new targets for running in existing containers (GCB).
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.
This is an early step to being able to build releases in Container Builder.
This simple solution to a complex problem brought to you by @javier-b-perez.
ref kubernetes/test-infra/issues/4958
ref kubernetes/test-infra/issues/3356
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