Enable specifying a remote image for analysis

[EricHripko]

Overview

Buildpacks can use a (previously published) remote image to speed up the build. As per https://github.com/moby/buildkit/issues/2097#issuecomment-830755638, this is conceptually similar to --cache-from functionality.

We should be able to lean on GetCacheImports API to enable specifying the remote image. UX will be roughly as follows:

• Docker CLI will need an extra argument:

  $ docker build --cache-from myregistry/myimage .

• Compose will need an extra attribute in the build section:

  services:
  webapp:
  build:
    context: .
    cache_from:
      - myregistry/myimage

  Since Buildpacks only support analysing a single image, only the first entry of registry type will be picked up.

CC @jromero for awareness

[jromero]

I'm assuming here that the cache-from can point to a local image as well with behaviour similar to docker run where it will pull it if it doesn't exist. I'm basing this theory based on this statement on the docs

BuildKit automatically pulls the image from the registry if needed.

I'm thinking about the rebuild scenario:

1. docker build -t my-app .
2. docker build -t my-app --cache-from my-app .