add: override artifactType

[wobito]

allow the config to setup the artifactType when the config mediatype is of empty type

[tri-adam]

One question and suggestion about the naming of the API. Thanks!

[wobito]

All your suggestions I initially had, but changed it.

Since I'm only setting the artifact type on the config I opted for the name structure.

Let's huddle tomorrow if you have time.

[tri-adam]

The code looks good, but as discussed, I think the resulting image manifest has the artifactType under the config block, rather than under the top level of the manifest?

For example, pkg/mutate/testdata/TestApply/SetConfigArtifactType/manifest.golden is currently:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
  "config": {
    "mediaType": "application/vnd.oci.empty.v1+json",
    "size": 2,
    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
    "artifactType": "application/vnd.sylabs.container"
  },
  "layers": [
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 3208,
      "digest": "sha256:7050e35b49f5e348c4809f5eff915842962cb813f32062d3bbdd35c750dd7d01"
    }
  ]
}

But I think, according to the spec this should be:

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
  "artifactType": "application/vnd.sylabs.container"
  "config": {
    "mediaType": "application/vnd.oci.empty.v1+json",
    "size": 2,
    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
  },
  "layers": [
    {
      "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
      "size": 3208,
      "digest": "sha256:7050e35b49f5e348c4809f5eff915842962cb813f32062d3bbdd35c750dd7d01"
    }
  ]
}

Potentially related: google/go-containerregistry#1537, google/go-containerregistry#1832

[wobito]

looks like its being worked on:
google/go-containerregistry#1931