c8d/image/inspect: Return Descriptor

[vvoland]

• follow up to: c8d/list: Return Descriptor #48861

Add the image target descriptor to the image inspect response.

- How to verify it

$ docker pull alpine
$ curl -s --unix-socket /var/run/docker.sock localhost/images/alpine/json | jq
{
  "Id": "sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a",
  "RepoTags": [
    "alpine:latest"
  ],
  "RepoDigests": [
    "alpine@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a"
  ],
  "Parent": "",
  "Comment": "buildkit.dockerfile.v0",
  "Created": "2024-09-06T12:05:36Z",
...
  "Descriptor": {
    "mediaType": "application/vnd.oci.image.index.v1+json",
    "digest": "sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a",
    "size": 9218
  }
}

$ docker pull arm64v8/alpine:3.10

$ curl -s --unix-socket /var/run/docker.sock localhost/images/arm64v8/alpine:3.10/json | jq

{
  "Id": "sha256:f73072b1ae04902f4d7511b834a1ea2d0475ddf6f87bd81f06e2b4590974a2e5",
  "RepoTags": [
    "arm64v8/alpine:3.10"
  ],
  "RepoDigests": [
    "arm64v8/alpine@sha256:f73072b1ae04902f4d7511b834a1ea2d0475ddf6f87bd81f06e2b4590974a2e5"
  ],
  "Parent": "",
  "Comment": "",
  "Created": "2021-04-14T18:43:17.827903143Z",
...
  "Descriptor": {
    "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
    "digest": "sha256:f73072b1ae04902f4d7511b834a1ea2d0475ddf6f87bd81f06e2b4590974a2e5",
    "size": 528
  }

- Description for the changelog

# Merge with the `GET /images/json`
containerd image store: `GET /images/json` and `GET /images/{name}/json` response now includes `Descriptor` field, which contains an OCI descriptor of the image target. The new field will only be populated if the daemon provides a multi-platform image store.

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

LGTM

should we have an integration test for this?

[vvoland]

Added a simple integration test

[thaJeztah]

LGTM

[laurazard]

LGTM