Build container images for all architectures and publish multi-arch manifests

[lentzi90]

We currently build and publish only amd64 images. There are other popular architectures, e.g. arm64, that we should support.
At least CAPM3 and IPAM already supports building for multiple architectures easily with make docker-build-all. BMO and Ironic-image will need some work though.

The script for building must also be updated so that it builds multiple architectures and then creates a multi-arch manifest from them.

• Support multiple architectures in BMO: Support building for multiple architectures metal3-io/baremetal-operator#1715
• Support multiple architectures in CAPM3: Support building for multiple architectures metal3-io/cluster-api-provider-metal3#1640
• Support multiple architectures in IPAM: Support building for multiple architectures metal3-io/ip-address-manager#517
• Support multiple architectures for Ironic: Support building & deploying for multiple architectures  metal3-io/ironic-image#502
• Build container images for multiple architectures and publish multi-arch manifests (this issue)

[tuminoid]

There is metal3-io/ip-address-manager#507 from week ago btw

[lentzi90]

Thanks, I had not seen that!

[lentzi90]

Hmm that means we have some work to do in CAPM3 also. It was actually a mistake that I put IPAM above. I meant to put ironic-image, which needs a bit more work I think. I'll update.