Adds extra build configuration to allow building multiple architectures

[iamkirkbater]

Adds additional configuration to build for multiple architectures.

I tried to add the functionality while keeping the original make targets like make build work the same as they do now for those who may be using those.

I highly recommend installing GNU parallel in order to build both at the same time. It takes ~20m to build amd64 on an arm64 laptop, FWIW.

Steps to test: (change IMAGE_REPOSITORY to your quay.io repo)

• ./.ci/build.sh
• make IMAGE_REPOSITORY=kbater TAG=latest-arm64 ARCHITECTURE=arm64 tag
• make IMAGE_REPOSITORY=kbater TAG=latest-amd64 ARCHITECTURE=amd64 tag
• make IMAGE_REPOSITORY=kbater TAG=latest-amd64 ARCHITECTURE=amd64 push
• make IMAGE_REPOSITORY=kbater TAG=latest-arm64 ARCHITECTURE=arm64 push
• make IMAGE_REPOSITORY=kbater build-manifest
• make IMAGE_REPOSITORY=kbater push-manifest
• podman manifest inspect quay.io/kbater/ocm-container:latest and validate that you see the two different architectures.

[iamkirkbater]

/hold

Let's wait to see if the nightly builds are working first before merging this and adding a new variable to the mix.

[iamkirkbater]

/hold cancel

[rendhalver]

/lgtm

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: iamkirkbater, rendhalver

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