Support buildx-style multi-arch manifest generation for better registry management

[Varorbc]

When using dotnet publish -t:PublishContainer to produce multi-arch images, the current implementation builds each architecture separately and then merges them into a manifest list. In container registries like Harbor, this results in the manifest and the individual architecture images being displayed at the same level, which makes management less intuitive.

By contrast, images produced with docker buildx build --platform ... --push appear in Harbor as a single manifest object. You can then drill down into the manifest to see the architecture-specific images. This hierarchy is easier to manage and aligns with how most registries present multi-arch images.

Is there any plan to support a buildx-style workflow in the .NET SDK, where dotnet publish could directly produce a multi-arch manifest in one step? This would simplify registry management and provide a more consistent experience across platforms.