[Investigation] Use HEAD request to determine if the digest had changed

[dfreilich]

Description

Currently, when we pack build, we attempt to pull some images (eg, we check whether there is a new version of your builder) over here. With DockerHub rate limits, that can be an issue.

Proposed solution

First use a HEAD request (perhaps using go-containerregistry remote.Head) to check whether the digest has changed, and only pull if it has.

Additional context

Slack discussion here

[jromero]

Relevant code:

• pack: /~https://github.com/buildpacks/pack/blob/main/internal/image/fetcher.go#L60-L68
• imgutil: /~https://github.com/buildpacks/imgutil/blob/main/remote/remote.go#L195-L202

[imjasonh]

It looks like pack's fetcher.go uses github.com/docker/docker/client, which I believe, being maintained by Docker, would already do the right thing and avoid unnecessarily-counted pulls if it can determine it can do so.

That leaves remote's Image.Found which pack calls before pullImage, which can be fixed to use remote.Head instead. Let me know if there's about this code path that I might be missing.

[jromero]

@imjasonh, got your imgutil feature incorporated in #908.

Based on your statement this is resolved. It's more that it delegates the lookup to the docker daemon that then should already do the right thing. Tomatoes potatoes.