Flips two defaults and adds setup-qemu-action so every caller that
doesn't override now produces a linux/amd64,linux/arm64 manifest list
out of the box, running on the new k8s-runner-amd64 in the primary
cluster (with QEMU emulating arm64 in the same dind sidecar).
Why now: the wectrl k8s cluster gained an x86 builder node, and the
warm-standby cluster in hel1 is already x86. With multi-arch images,
both clusters can pull the same manifest — no more per-arch divergence,
no more rebuilds when migrating workloads between clusters.
Callers can still pin to a single arch (`platforms: linux/arm64`) or
keep the existing arm runner (`runner: ubuntu-latest`) when an ERPNext-
sized build would be unacceptable under QEMU. Most JS/Astro/static
images aren't bottlenecked by the emulated leg.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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