Add ko extract to extract ko-built image references from rendered YAML

[imjasonh]

@mattmoor @jonjohnsonjr

Tekton currently does this with a custom-built Python script, koparse, and first-party support in ko itself could be useful.

[imjasonh]

#406
#358 (comment)

[codecov-commenter]

Codecov Report

Merging #443 (d5415e8) into main (2dced74) will decrease coverage by 0.47%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
- Coverage   53.25%   52.78%   -0.48%     
==========================================
  Files          37       38       +1     
  Lines        1842     1887      +45     
==========================================
+ Hits          981      996      +15     
- Misses        710      739      +29     
- Partials      151      152       +1     

Impacted Files	Coverage Δ
pkg/commands/extract.go	31.81% <31.81%> (ø)
pkg/commands/commands.go	76.92% <100.00%> (+1.92%)	⬆️

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[jonjohnsonjr]

I feel like it might be more useful to have a flag where ko can spit out anything it builds? I think skaffold does something similar with --file-output.

Doing this with a resolved yaml file means we'll pick up anything in KO_DOCKER_REPO, not just things that we built.

[imjasonh]

I feel like it might be more useful to have a flag where ko can spit out anything it builds? I think skaffold does something similar with --file-output.

Doing this with a resolved yaml file means we'll pick up anything in KO_DOCKER_REPO, not just things that we built.

That would be fine too, we'd just need to settle on an output format. OCI layout? Newline-delimited image refs@digest?

ko extract as it is in this PR is imperfect, sure, but it would do everything we need for Tekton and Knative (I think) -- I can't think of a use case where someone would have other KO_DOCKER_REPO-prefixed strings in their YAML that we shouldn't report. Maybe we could have ko resolve emit fully-qualified registry URLs to prevent the KO_DOCKER_REPO=username case, which could lead to trouble here.

We could document its limitations a bit better, if that's a concern.

[jonjohnsonjr]

We could document its limitations a bit better, if that's a concern.

Yeah this is my main issue. The command is fine, we should just be more transparent about what it's doing.

[imjasonh]

Closing this until it can be rebased on #453