fix: make IMAGE_TAG available in buildArgs when used in docker FROM

[abe-winter]

Related:

• related fix for kaniko Make IMAGE_REPO, IMAGE_NAME, IMAGE_TAG available in build.artifacts[i].docker.buildArgs #4295
• related fix fix: Fill IMAGE_TAG,etc on Docker builds #7788

Description
Apologies if I am simply misunderstanding something and this can be fixed w/out a code change. I am certainly not fully wrapping my head around this codebase.

This PR adds the ability to use IMAGE_TAG (IMAGE_REPO etc) in the docker buildArgs section, specifically when the build arg is then consumed in the Dockerfile FROM. On current stable and main, this case fails in the depList / SingleArtifactDependencies phase, with logs like:

Checking cache...

• base: Found Remotely
• child: Error checking cache.

getting hash for artifact "child": getting dependencies for "child": parsing ONBUILD instructions: retrieving image "gcr.io/headsdown/base:": parsing reference "gcr.io/headsdown/base:": could not parse reference: gcr.io/headsdown/base:

Note that when used with dependencies (as below), this seems to only work reliably if I run skaffold build && skaffold run. skaffold run by itself will sometimes fail with 'image not found remotely'.

Repro

Stripped-down skaffold.yaml:

build:
  local:
    useBuildkit: true
  artifacts:
    - image: base
      docker:
        dockerfile: Dockerfile
    - image: child
      requires:
      - image: base
      docker:
        dockerfile: child/Dockerfile
        buildArgs:
          IMAGE_TAG: '{{.IMAGE_TAG}}'

Top of child/Dockerfile:

# syntax=docker/dockerfile:1
ARG IMAGE_TAG
FROM gcr.io/PROJECTNAME/base:$IMAGE_TAG

tldr the second artifact depends on the first and then uses it as a base image

User facing changes
I think the docs already imply this is possible

Follow-up Work
No idea if I'm conforming to house style -- if accepted, guessing someone on the team will need to clean up or at least assign me cleanup. I would normally have submitted a bug rather than a PR for something like this, but I didn't understand why my template param was missing until I'd already solved the problem.

If this is something useful, happy to add tests etc.

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