fix: hack to output OCI v1.1 artifact

[tri-adam]

Description of the Pull Request (PR):

Data containers in Singularity use an empty JSON document as their "config". Thus, based on the OCI v1.1 release notes, they're a good candidate to make use of the artifactType field.

The google/go-containerregistry does not currently include artifactType in v1.Manifest, as per the OCI v1.1 specification. There is an open PR to address that (google/go-containerregistry#1931) but it hasn't had any movement in a while. This PR works around the lack of support until that field is added in the google/go-containerregistry module.

This fixes or addresses the following GitHub issues:

• Fixes #

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[wobito]

just noticed one thing

[dtrudg]

LGTM. Many thanks for this!

[tri-adam]

Just had one more thought this morning, we are setting the artifactType to application/vnd.sylabs.data-container.config.v1+json... but the artifact itself is not a config, nor is it JSON document? The layer media types describe the format of the layer, so I don't think we need to describe that here, but perhaps this should just be something like application/vnd.sylabs.data-container.v1?

[dtrudg]

Just had one more thought this morning, we are setting the artifactType to application/vnd.sylabs.data-container.config.v1+json... but the artifact itself is not a config, nor is it JSON document? The layer media types describe the format of the layer, so I don't think we need to describe that here, but perhaps this should just be something like application/vnd.sylabs.data-container.v1?

Ahh, yeah. That would make sense without the .config 👍