Support building for multiple architectures

[lentzi90]

User Story

As a developer/user/operator I would like to use pre-built ironic 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.
I suggest we look at how CAPI/CAPM3 has structured the Makefile with a docker-build-all target for building all supported architectures. It will probably be a bit different for Ironic though, since it is not written in Go.

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.

/kind feature

[dtantsur]

This is going to be a huge can of worms :) Start with #437 if you feel courageous.

[Rozzii]

/tirage accepted
After we have sorted out our CI "restructuring" we could ask for some Oracle arm compute power from CNCF to have some ARM CI.
/help

[metal3-io-bot]

@Rozzii:
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In response to this:

/tirage accepted
After we have sorted out our CI "restructuring" we could ask for some Oracle arm compute power from CNCF to have some ARM CI.
/help

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[Rozzii]

/triage accepted

[metal3-io-bot]

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[tuminoid]

/remove-lifecycle stale