OCI base image support is likely to be fishy or even not working

[chanseokoh]

We does accept OCI manifest and read it into OCIManifestTemplate in memory.

• ManifestPuller

    if (schemaVersion == 2) {
      // 'schemaVersion' of 2 can be either Docker V2.2 or OCI.
      String mediaType = node.get("mediaType").asText();
      ...
      if (OCIManifestTemplate.MANIFEST_MEDIA_TYPE.equals(mediaType)) {
        return manifestTemplateClass.cast(
            JsonTemplateMapper.readJson(jsonString, OCIManifestTemplate.class));
      }

readJson() above returns an OCIManifestTemplate object (which is cast into the super ManifestTemplate by .cast()).

But, when we re-cast the in-memory object, we case it into either V21ManifestTemplate or V22ManifestTemplate.

• PullBaseImageStep

    ManifestTemplate manifestTemplate =
        registryClient.pullManifest(buildConfiguration.getBaseImageConfiguration().getImageTag());

    // TODO: Make schema version be enum.
    switch (manifestTemplate.getSchemaVersion()) {
      case 1:
        V21ManifestTemplate v21ManifestTemplate = (V21ManifestTemplate) manifestTemplate;
        ...

      case 2:
        V22ManifestTemplate v22ManifestTemplate = (V22ManifestTemplate) manifestTemplate;
        ...

So although I haven't tested myself, I think Jib will throw class cast exception.

Also, we should have a "default" case in the above switch.