fix(preview): build preview images for arm64 too

[nicacioliveira]

A running preview died on a spot reclaim.

The studio-preview node pool schedules on amd64 and arm64 so Karpenter can take whatever spot capacity exists. The preview images were amd64 only. Karpenter replaced an r8in.large with an r7gd.large (Graviton), and every pod came back as:

exec /usr/local/bin/bun: exec format error

The release images have been multi-arch (linux/amd64 + linux/arm64) all along — previews were the outlier, on the stated grounds that "the preview cluster is amd64". The node pool that shipped alongside them does not guarantee that, and did not honour it on the first reclaim.

Why QEMU and not the release matrix

release-studio.yaml builds each architecture on a native runner and stitches the digests with buildx imagetools create. That is the right shape for the release path. Here it would mean a second job, digest artifacts and a manifest-merge step to save ~51s — by the release workflow's own numbers, emulated arm64 costs ~64s against ~13s native for the tarball-install layer. That is roughly 9% of a ~9-minute preview build. Not worth the machinery for a disposable environment.

cache-from still reads the release scopes, so the apt / useradd / duckdb layers stay cache hits.

Assertion

Nothing in a Helm render can observe an image manifest, so helm-test.yml now checks the build workflow directly: if the pool takes arm64, the build has to target it.

Alternative considered

Pinning the node pool to amd64 is one line and also fixes it — but it gives up Graviton, which is usually ~20% cheaper, and narrows the instance set on a spot-only pool, which raises the reclaim rate on exactly the workload that just proved it is sensitive to reclaims.

After merge

Existing preview images stay single-arch; they are rebuilt on the next push to their PR. #6205 is currently down for this reason and recovers on its next build.

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Summary by cubic

Builds preview images for both amd64 and arm64 by building each architecture on native runners and publishing a merged manifest. Previously previews shipped amd64-only images; when pods landed on Graviton, they failed with “exec format error.”

• .github/workflows/preview-build.yaml: split into package (tarball), docker matrix for linux/amd64 and linux/arm64 on native runners, push by digest, then publish stitches digests with docker buildx imagetools create and comments the preview URL. Image tags exist only after the manifest merge.
• .github/workflows/helm-test.yml: assert the arm64 runner is used and that manifests are merged, since Helm renders cannot observe image manifests.

Rollout

• No app changes. New preview builds publish multi-arch images.
• Previews with amd64-only images must rebuild; push a new commit to the PR to trigger a build.

^{Written for commit faad048. Summary will update on new commits.}