containertool: Store an index, pointing to the image manifest

[euanh]

Motivation

A multiarch image consists of an index pointing to one or more architecture-specific manifests. containertool currently builds a single architecture image, which can be pushed using just a manifest, but including the index as well is a step towards being able to build multiarch images in future.

Modifications

• Add putIndex operation to the ImageDestination protocol and implement it in RegistryClient
• Build an index in publishContainerImage and print its reference on standard output, instead of the reference of the manifest.

Result

Although it still builds single-architecture images, a containertool image will start with an index pointing to a manifest, instead of a bare manifest.

Test Plan

All existing tests continue to pass.