Build pulls referenced images when all we need is a digest

When porter build runs, during the step where we generate the bundle.json file we need to fetch the repository digest for all referenced images.  Right now we do a full pull on each image, which for lots of images or large images is terribly slow. We should only retrieve the data we need, the repository digest.

See https://github.com/getporter/porter/discussions/2565