[FEATURE] build multi-arch images in parallel

[akostadinov]

Is your feature request related to a problem? Please describe.

When building multi-arch image with buildx (docker/build-push-action@v2), builds complete much quicker than with redhat-actions/buildah-build@v2.

It seems to me that buildx action performs builds in parallel and this saving significant time from the builds. As an example, my multi-arch build with buildx takes 41 minutes and with buildah - 1h36m

Describe the solution you'd like

build images in parallel and provide a configuration option to run in sequence when desired.