fix(ci): test multi-arch Docker build in preflight before releasing

Previously, preflight only built amd64 images, so multi-arch failures
(like the QEMU timeout in 5.0.5) weren't caught until after the
release was published.

Now preflight builds linux/amd64,linux/arm64 staging images. If
multi-arch build fails, the release pipeline stops before publishing.

Combined with the Dockerfile fix (forcing amd64 for build stages),
this ensures Docker build issues are caught early.

.github/workflows/create-release.yml

@@ -170,12 +170,12 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Build and push staging Docker images
+      - name: Build and push staging Docker images (multi-arch)
         uses: docker/build-push-action@v6
         with:
           context: .
           target: runtime
-          platforms: linux/amd64  # amd64-only for faster preflight; multi-arch happens in release job
+          platforms: linux/amd64,linux/arm64  # Multi-arch to catch build issues before release
           push: true
           provenance: false
           cache-from: type=registry,ref=ghcr.io/${{ github.repository_owner }}/pulse:buildcache
@@ -185,13 +185,13 @@ jobs:
           tags: |
             ghcr.io/${{ github.repository_owner }}/pulse:staging-${{ needs.extract_version.outputs.tag }}
 
-      - name: Build and push staging Docker agent image
+      - name: Build and push staging Docker agent image (multi-arch)
         uses: docker/build-push-action@v6
         with:
           context: .
           file: ./Dockerfile
           target: agent_runtime
-          platforms: linux/amd64  # amd64-only for faster preflight; multi-arch happens in release job
+          platforms: linux/amd64,linux/arm64  # Multi-arch to catch build issues before release
           push: true
           provenance: false
           cache-from: type=registry,ref=ghcr.io/${{ github.repository_owner }}/pulse-docker-agent:buildcache