We needed ARM64 containers. Our Python services run on mixed infrastructure: amd64 in CI and some production clusters, arm64 on newer nodes. Building on one arch and emulating the other with QEMU was painfully slow and broke native extensions. So we added proper multi-arch builds to our GitHub Actions CI.

It took a week to get right. Then five more fixes over two weeks as each failure mode revealed itself in production. This is what went wrong and how we fixed it.