Feat: output grouped digest

[waybackarchiver]

Hi guys,

Really like this action. I discovered cosign requires digest, but this action does not group all of the image digest output, which would be preferred if supported.

Relates to sigstore/cosign#2047

Thanks.

[crazy-max]

metadata output already contains containerimage.digest for all built targets: https://github.com/docker/bake-action/actions/runs/5222125090/jobs/9427226905#step:5:234

  {
    "t1": {
      "containerimage.buildinfo": {
        "frontend": "gateway.v0",
        "attrs": {
          "cmdline": "docker/dockerfile:1",
          "filename": "Dockerfile",
          "source": "docker/dockerfile:1",
          "target": "t1",
          "vcs:revision": "eb348b10d62f5f6e9386118f8dc00e45aac33aa7",
          "vcs:source": "https://github.com/docker/bake-action"
        },
        "sources": [
          {
            "type": "docker-image",
            "ref": "docker.io/docker/dockerfile:1",
            "pin": "sha256:39b85bbfa7536a5feceb7372a0817649ecb2724562a38360f4d6a7782a409b14"
          },
          {
            "type": "docker-image",
            "ref": "docker.io/library/busybox:latest",
            "pin": "sha256:560af6915bfc8d7630e50e212e08242d37b63bd5c1ccf9bd4acccf116e262d5b"
          }
        ]
      },
      "containerimage.descriptor": {
        "mediaType": "application/vnd.oci.image.index.v1+json",
        "digest": "sha256:38810b25e60cc4b990aaaac46d4aac25674b062a130eae5c3727ed7276e0eb77",
        "size": 855
      },
      "containerimage.digest": "sha256:38810b25e60cc4b990aaaac46d4aac25674b062a130eae5c3727ed7276e0eb77",
      "image.name": "localhost:5000/name/app:t1"
    },
    "t2": {
      "containerimage.buildinfo": {
        "frontend": "gateway.v0",
        "attrs": {
          "cmdline": "docker/dockerfile:1",
          "filename": "Dockerfile",
          "source": "docker/dockerfile:1",
          "target": "t2",
          "vcs:revision": "eb348b10d62f5f6e9386118f8dc00e45aac33aa7",
          "vcs:source": "https://github.com/docker/bake-action"
        },
        "sources": [
          {
            "type": "docker-image",
            "ref": "docker.io/docker/dockerfile:1",
            "pin": "sha256:39b85bbfa7536a5feceb7372a0817649ecb2724562a38360f4d6a7782a409b14"
          },
          {
            "type": "docker-image",
            "ref": "docker.io/library/busybox:latest",
            "pin": "sha256:560af6915bfc8d7630e50e212e08242d37b63bd5c1ccf9bd4acccf116e262d5b"
          }
        ]
      },
      "containerimage.descriptor": {
        "mediaType": "application/vnd.oci.image.index.v1+json",
        "digest": "sha256:0212cde084fda12a3b7478248fad57e121727eac6492c11abad4f5a1288b82e5",
        "size": 855
      },
      "containerimage.digest": "sha256:0212cde084fda12a3b7478248fad57e121727eac6492c11abad4f5a1288b82e5",
      "image.name": "localhost:5000/name/app:t2"
    }
  }