TARGETARCH automatically set in docker but not podman

[ericcurtin]

Feature request description

When building Dockerfiles like this one, it builds with docker and not podman. docker sets targetarch automatically while podman doesn't:

https://github.com/docker/model-runner/blob/main/Dockerfile

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Have you considered any alternatives?

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Additional context

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[ericcurtin]

--platform linux/amd64 might make a difference to docker build.

[afbjorklund]

Using --build-arg TARGETARCH ... as a workaround, to fulfill ARG TARGETARCH

Also BUILDPLATFORM is missing, for lines like FROM --platform=$BUILDPLATFORM

[baude]

@nalind ptal , is this a bug or intentful ?

[nalind]

The docs are subtle on whether or not built-in args need to be declared before they're used in the heading, which is a requirement in regular build stages, i.e. at any point after a FROM instruction. The example at https://github.com/docker/model-runner/blob/main/Dockerfile suggests that they shouldn't need to be declared before using them in the heading, but our implementation isn't relaxing its requirement for the heading. That's a bug. Until we fix that, declaring it before use, like you'd have to do in a particular stage, is harmless, and it appears to be enough to keep the build going.

[baude]

@nalind should we transfer this to buildah?

[nalind]

Sure. I expect the bug fix will go either there, in https://github.com/openshift/imagebuilder, or in both.

[NotSoFancyName]

Could I work on it?

[nalind]

Could I work on it?

Go for it.

[NotSoFancyName]

/assign