Always produce OCI images and indexes

[imjasonh]

Followup from #354

Before this change, if the base image was a Docker manifest list, it could not be annotated with base image information. With this change, ko always produces OCI images and indexes, which can be annotated.

Verified manually as well:

$ crane manifest $(go run ./ publish ./test --platform=all) | jq '.annotations'
...
{
  "org.opencontainers.image.base.digest": "sha256:7cb5539ebb7b99352d736ed97668060cee123285f01705b910891acdf7d945e3",
  "org.opencontainers.image.base.name": "gcr.io/distroless/static:nonroot"
}                                                                                                                                                      

[jonjohnsonjr]

My only reluctance here is breaking folks that push to old registries that won't support OCI. I guess they could just use an old version of ko... wdyt?

[imjasonh]

My only reluctance here is breaking folks that push to old registries that won't support OCI. I guess they could just use an old version of ko... wdyt?

Yeah... we could have an --oci flag that defaults true?

[jonjohnsonjr]

Yeah... we could have an --oci flag that defaults true?

I'd be ok with deferring this until someone complains.

[codecov-commenter]

Codecov Report

Merging #449 (8c089af) into main (2adcadd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #449   +/-   ##
=======================================
  Coverage   53.25%   53.26%           
=======================================
  Files          36       36           
  Lines        1842     1838    -4     
=======================================
- Hits          981      979    -2     
+ Misses        710      709    -1     
+ Partials      151      150    -1     

Impacted Files	Coverage Δ
pkg/build/gobuild.go	58.51% <100.00%> (+0.07%)	⬆️

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