[9.3] (backport #12363) Allow locally building multiplatform docker images

[mergify]

What does this PR do?

It allows building cross-platform docker images. We can already cross-build agent binaries, but for docker images we had a check specifically limiting them to the host architecture. However, buildx makes this pretty straightforward. This PR adds a check for buildx platform availability, and uses it if the host arch doesn't match the requested container image arch.

The cross-built container images gets the arch appended to the tag, so when building main for arm64 on a amd64 host, we get the 9.0.0-SNAPSHOT-arm64 tag. The native image gets a tag without the suffix, which is a bit inconsistent, but preserves the current behaviour.

This doesn't change anything for our CI, which builds arm images on arm hosts, but it makes cross-building locally much simpler.

This is a resubmission of #6256, which ended up having to be reverted due to unexpected breakage in unified release. We're now much better at testing that workflow in our PR CI, so this should be much safer.

Why is it important?

It's useful sometimes to be able to locally create a cross-platform manifest. I was troubleshooting some issues in our internal clusters, and wanted the SRE team to test a custom image I built, and had to hack around the build system to make it work.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas

How to test this PR locally

DEV=true SNAPSHOT=true PACKAGES=docker DOCKER_VARIANTS=basic PLATFORMS="linux/arm64,linux/amd64" EXTERNAL=true mage package

Should build two docker images.

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This is an automatic backport of pull request #12363 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @swiatekm? 🙏

[swiatekm]

FYI, I'm waiting for the 9.3.0 release to merge this. There's no rush, and I want to avoid doing anything that could impact the unified release process.