Codify image name defaults

[jromero]

As a Platform implementer, it would be nice to codify the currently implicit contract surrounding defaults in image names.

As of this writing, the lifecycle uses GGCR registry parsing with WeakValidation which would set certain defaults as described below.

From GGCR:

For defaulting, we interpret "ubuntu" as "index.docker.io/library/ubuntu:latest" because we add the missing repo "library", the missing registry "index.docker.io", and the missing tag "latest".

This behaviour should be explicitly called out in the spec so that CNB Platforms can be assured that the behaviour would be applied regardless of any specific Lifecycle implementation.