Add a tag-only flag to publisher.

[chhsia0]

The --tag-only flag resolves images into tag-only references. This flag is useful when digests are not preserved when images are repopulated. It can only be used when a single tag is specified, and the tag is not latest.

[imjasonh]

I'm not sure I understand the use case, in what cases are references by digest not preferred?

Images by tag are problematic because the tag can change and you can get unexpected/inconsistent behavior across replicas/nodes.

[jonjohnsonjr]

I can imagine a couple situations where this could be useful, e.g. when writing to the docker daemon, but not when pushing to the registry. Agree with Jason, I'd like to understand the problem a little more.

[codecov-io]

Codecov Report

Merging #332 (ab03af6) into main (0b96f41) will increase coverage by 0.14%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##             main     #332      +/-   ##
==========================================
+ Coverage   38.24%   38.39%   +0.14%     
==========================================
  Files          33       33              
  Lines        1527     1542      +15     
==========================================
+ Hits          584      592       +8     
- Misses        849      853       +4     
- Partials       94       97       +3     

Impacted Files	Coverage Δ
pkg/commands/resolver.go	11.60% <0.00%> (-0.07%)	⬇️
pkg/publish/default.go	61.84% <45.45%> (-2.78%)	⬇️
pkg/publish/options.go	28.12% <100.00%> (+7.43%)	⬆️

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[chhsia0]

Sure I could explain more. Our use case is to use ko to generate manifests that supports air-gapped environments, where we repopulate necessary images to a local registry or containerd from tarballs (similar to docker load). However, such repopulation does not preserve image digests, so we have to use tags to refer to a certain image version in the manifests so they can be deployed in both a normal and an air-gapped environment.

[chhsia0]

In general I agree that image digests are preferred, just that it doesn't work in this particular use case.

I also put a constraint for this tag to disallow people from generating image references that uses the (commonly) moving latest tag.

[jonjohnsonjr]

I also put a constraint for this tag to disallow people from generating image references that uses the (commonly) moving latest tag.

I'm not sure if we want to be that opinionated about latest. I generally agree with you, but it feels like an artificial constraint. That being said, it is easier to relax this if people complain than it would be to introduce this constraint and break people who rely on it, so I don't have a strong opinion.

I'm ok to merge unless @imjasonh has objections

[imjasonh]

Sgtm