This sounds cool. At my current job we do multi-arch builds in GCP Cloud Build using `docker buildx`. The native `amd64` builds take like 15 minutes and the `arm64` component takes well over an hour with occasional time outs. We use the `--cache-to/--cache-from` to speed up repeated builds. If we can't wait the hour+ for the emulation build to run, we have a `seed-mac-arm64.sh` script that a developer (i.e., me) can run on their M1 mac to do the `arm64` half of the build, which is then picked up by GCP Cloud Build. It's a small pain, but helps us in a pinch.

This solution sounds far easier!