Support different container registry to push and pull

[rgaiacs]

Proposed change

In some advanced use cases, the operator might want to push to one container registry and pull from another container registry. For example, the operator want to push to a public container registry like https://hub.docker.com/ and pull from a private proxy cache.

This will require

• Add extra option in Helm chart

  config:
    BinderHub:
      use_registry: true
      image_prefix_to_push: "your-push-registry.io/<prefix>-"
      image_prefix_to_pull: "your-pull0registry.io/<prefix>-"

• Support new option when building container

• Support new option when contacting JupyterHub

Who would use this feature?

@arnim and @rgaiacs want to use it at the mybinder.org server operated by GESIS.