align with OCI artifact best practices

[ndeloof]

What I did
better align with OCI artifact best practices (https://oras.land/docs/concepts/artifact/):

• artifact blob layer is pushed with mediatype application/vnd.oci.image.layer.v1.tar
• org.opencontainers.image.created annotation is set

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[neersighted]

I don't think that page ORAS represents best practices; in particular there is no need to use a layer tar mediaType when that is not actually the format of the data.

It's worth noting that they also show a custom layer mediaType in their "with config" example -- I suspect the overloading of the layer mediaType is merely an error in updating the page.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (6ab41d6) 57.54% compared to head (f659896) 57.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11120      +/-   ##
==========================================
- Coverage   57.54%   57.44%   -0.10%     
==========================================
  Files         129      129              
  Lines       11272    11276       +4     
==========================================
- Hits         6486     6478       -8     
- Misses       4156     4166      +10     
- Partials      630      632       +2     

Files	Coverage Δ
pkg/compose/publish.go	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

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[ndeloof]

@neersighted right, would you recommend a better "source of truth" ? I hardly can find documentation on OCI artifacts, Google 1st result being archived https://github.com/opencontainers/artifacts 😅

[neersighted]

https://github.com/opencontainers/image-spec/blob/main/manifest.md#guidelines-for-artifact-usage

I'm also working on trying to make more of this obvious in the spec, e.g. at opencontainers/image-spec#1141. Reviews of that PR/suggestions for further content or structure improvements are welcome.

[neersighted]

Oh! And if I wasn't clear -- I think it's still worth carrying the annotation change; just not the change to the layer mediaType.

[ndeloof]

sure, I plan to update this PR with a few other fixes, so making this one a draft

[ndeloof]

"reopen" action failing, will create another one 😅