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

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.

Start with the `dotnet publish -t:PublishContainer` entry point and compare its current multi-architecture workflow with `docker buildx build --platform ... --push`. Review how the generated manifest and architecture-specific images are pushed to registries such as Harbor. Done means a documented or implemented buildx-style workflow that presents one drill-down multi-arch manifest object.

Written by the indexing model from the issue text.