ci(preview): build Docker previews for arm64 as well as amd64

The preview image was amd64-only while the release and stable images are built for linux/amd64,linux/arm64, so a preview tag could not stand in for socketdev/cli:latest on arm64 hosts without emulation. Match the release arch matrix and enable QEMU so the arm64 layer can be built on an amd64 runner. Previews are opt-in via label, so the extra build time is an acceptable tradeoff for making the tag a drop-in replacement.

.github/workflows/pr-preview.yml

@@ -254,7 +254,8 @@ jobs:
  - name: Set up Docker publishing
  uses: ./.CloneAGC/actions/setup-docker
  with:
- enable-qemu: "false"
+ # QEMU is required to build the arm64 layer on an amd64 runner.
+ enable-qemu: "true"
  dockerhub-username: ${{ secrets.DOCKERHUB_USERNAME }}
  dockerhub-token: ${{ secrets.DOCKERHUB_TOKEN }}
 
@@ -268,7 +269,10 @@ jobs:
  file: Dockerfile.preview
  push: true
  pull: true
- platforms: linux/amd64
+ # Match the arch matrix of the release and stable images so a preview is a
+ # drop-in replacement for socketdev/cli:latest on arm64 runners too. The
+ # arm64 layer builds under emulation, so expect roughly double the runtime.
+ platforms: linux/amd64,linux/arm64
  tags: socketdev/cli:pr-${{ needs.context.outputs.pr_number }}
  build-args: |
  SDK_PREVIEW_VERSION=${{ inputs.sdk_preview_version }}