Container Support

Because we modeled some ManifestList fields on the through-table ManifestListManifest, they are not currently visible via the REST API.

From the Docker v2 API doc: https://docs.docker.com/registry/spec/manifest-v2-2/

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
  "manifests": [
    {
      "mediaType": "application/vnd.docker.image.manifest.v2+json",
      "size": 7143,
      "digest": "sha256:e692418e4cbaf90ca69d05a66403747baa33ee08806650b51fab815ad7fc331f",
      "platform": {
        "architecture": "ppc64le",
        "os": "linux",
      }
    },
    {
      "mediaType": "application/vnd.docker.image.manifest.v2+json",
      "size": 7682,
      "digest": "sha256:5b0bcabd1ed22e9fb1310cf6c2dec7cdef19f0ad69efa1f392e94a4333501270",
      "platform": {
        "architecture": "amd64",
        "os": "linux",
        "features": [
          "sse4"
        ]
      }
    }
  ]
}

Here's what users can currently see via the REST API:

~/d/devel ❯ http --auth admin:admin http://pulp3:8000/pulp/api/v3/content/docker/manifest-lists/1/

{
    "_artifact": {
        "_href": "/pulp/api/v3/artifacts/1/",
        "size": 1746
    },
    "_created": "2019-01-10T15:31:33.238122Z",
    "_href": "/pulp/api/v3/content/docker/manifest-lists/1/",
    "_type": "pulp_docker.manifest-list",
    "digest": "sha256:e6ac6227ccc94fd735278b4ee97dd029c7bcf4eba385c6998eb7b40b006c9b46",
    "manifests": [
        "/pulp/api/v3/content/docker/manifests/85/",
        "/pulp/api/v3/content/docker/manifests/83/",
        "/pulp/api/v3/content/docker/manifests/122/",
        "/pulp/api/v3/content/docker/manifests/84/",
        "/pulp/api/v3/content/docker/manifests/82/"
    ],
    "media_type": "application/vnd.docker.distribution.manifest.list.v2+json",
    "schema_version": 2
}

From ManifestListManifest, we need to be able to view all the fields:

1. architecture
2. os
3. os_version
4. os_features
5. features
6. variant

We it also might be helpful to be able to view (on the ManifestList) some fields that exist on Manifest:

1. media type
2. digest
3. schema version
4. size of Manifest artifact