Bootstrapper release instructions need to explain how to build at appropriate commit

[jlewi]

The bootstrapper release instructions need to explain how to bake the correct version of the repository for our registry into the container image.

https://github.com/kubeflow/kubeflow/blob/master/docs_dev/releasing.md

See #1018. for example.

The process should probably look something like the following

• Build a bootstrapper image that uses the version of the ksonnet registry on the release branch at the commit desired

• Submit a PR to the release branch updating references to bootstrapper image.

As @pdmack Suggested it might help if we tag our images using RC tags e.g

gcr.io/kubeflow-images-public:0.2.0-rc.0.1

This way we know what the tag will be before the image is built so we don't have to commit a PR after building the image. I think Docker Images have some form of metadata that we could use to store the commit so we know the code from which the images were built.

[jlewi]

We want to stop baking the registry into the bootstrapper image #1155 .

So this issue is obsolete.