bake: support manifests

Description

Hi,

I would like to use buildx bake to build windows images.

Images Docker images has the requirement that OS and host matches. In conclusion, I need a docker images for WIndows Server 2019 and Windows Server 2022.

Pushing images to the same tag will create a dedicated manifest of each target and overrides the other targets.

For example

target "windows-nanoserver-ltsc2019" {
  inherits = ["docker-metadata-action"]
  tags = ["app:latest"]

  platforms = ["windows/amd64"]

  target = "windows"
  args = {
    NANOSERVER_VERSION = "ltsc2019"
  }
}

target "windows-nanoserver-ltsc2022" {
  inherits = ["docker-metadata-action"]
  tags = ["app:latest"]

  platforms = ["windows/amd64"]

  target = "windows"
  args = {
    NANOSERVER_VERSION = "ltsc2022"
  }
}

Push targets push images to app:latest, after completion, only one image will be present in the manifest. I would be nice to setup an target which covers the manifest generation.

Start at the buildx bake workflow and its handling of push targets, then trace how repeated pushes to the same tag are represented. The issue is done when bake can coordinate the Windows Server 2019 and 2022 targets so the shared tag contains both images in one manifest.