stage_executor: Add support for inline FROM --platform= within Containerfile/Dockerfile

[flouthoc]

See: https://docs.docker.com/engine/reference/builder/#from

Allows end users to configure executor's OS, ARCH,VARIANT via inline --platform.

Usage

 FROM --platform=linux/arm64 alpine
 RUN uname -a

---

While this allows executor to pull base images with custom
OS, ARCH, VARIANT it still allows end-users to tag images with
different format if they need to.

For example

Doing a buildah build --platform linux/amd64 -t test . on below containerfile.

FROM --platform=linux/arm64 alpine
RUN uname -a

Would result in tagging image as linux/amd64 while base layer would
be pulled for linux/arm64.

Closes: #3712
Closes: #3261

[flouthoc]

Please don't review this PR yet as this PR is a implementation of : openshift/imagebuilder#212 so lets wait for that.

[flouthoc]

@nalind @rhatdan PTAL. Also modified tests to also check if qemu-user-static is already there on platform as @edsantiago suggested in the last PR.

[flouthoc]

This PR closes two existing open issues.

[rhatdan]

LGTM
@nalind @vrothberg @TomSweeneyRedHat @ashley-cui @umohnani8 PTAL

As soon as this is merged, we can cut a new release.

[openshift-ci]

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[nalind]

Might be simpler to grep the output of buildah version's BuildPlatform, which provides the entire value you're looking to use.

[flouthoc]

@nalind I need to toggle only arch at certain places that's why i preferred this. Hope this is not a blocker.

[rhatdan]

/lgtm