Publish a multi-architecture Corral web container image

[eseiker]

Problem

The published ghcr.io/tuna-os/corral:latest image is currently amd64-only even though Corral's rolling CLI binaries and marketplace plugins already support both amd64 and arm64.

The container workflow hard-codes GOARCH=amd64 and downloads amd64-only kubectl and virtctl binaries before using a single-platform Docker build. This prevents the in-cluster web UI from running natively on arm64 Kubernetes nodes. The image publication job is skipped on pull requests, so architecture regressions are not caught before merge.

Proposed change

• Prepare Corral, kubectl, and virtctl artifacts for both amd64 and arm64.
• Select the matching artifacts with BuildKit's target architecture.
• Build and publish one multi-platform image containing linux/amd64 and linux/arm64 manifests.
• Build both platforms on pull requests without publishing them.

Acceptance criteria

• The container build succeeds for both linux/amd64 and linux/arm64.
• Each platform contains architecture-matching Corral, kubectl, and virtctl binaries.
• Main-branch publication preserves the existing latest and short-SHA tags.
• Pull requests validate both container platforms without requiring package write access.