Support building for multiple architectures

[lentzi90]

User Story

As a developer/user/operator I would like to use pre-built BMO images on multiple architectures because it is convenient (compared to building everything yourself).

Detailed Description
We currently only publish amd64 container images, but there are other popular architectures. Go has good support for building for other architectures also, so there is no need for special hardware for testing this.
I suggest we look at how CAPI/CAPM3 has structured the Makefile with a docker-build-all target for building all supported architectures. This should be simple to implement for us as well.

Once we can build these images, it should be fairly trivial to also make CI build and publish them together with a multi-arch manifest for easy consumption.

Anything else you would like to add:

/kind feature

[Rozzii]

/triage accepted
/help

[metal3-io-bot]

@Rozzii:
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In response to this:

/triage accepted
/help

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[metal3-io-bot]

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[tuminoid]

/remove-lifecycle stale