Authenticate for CacheFrom via input credentials on Image

[guineveresaenger]

This PR refactors the registry authentication logic into its own function.
We can now authenticate as-needed for either a registry pull cache or for pushing an image. We reuse the fallback logic to enable us to refer to resource-based registry credentials when provided, and attempt to use ambient credentials when they are not otherwise provided.
Because of the way auth objects get passed, failing to authenticate to a registry-as-cache will not block the build; Docker will attempt to use the local cache or otherwise build from scratch.

• First pass at auth logic from the image inputs
• Refactor auth functionality into function to reuse
• Log into registry only once
• remove debug statements
• Add documentation to cacheFrom

Fixes #495

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[AaronFriel]

Clean refactor, much easier to follow the dockerBuild function now.