fix: host builder can push to OCP internal registry

[matejvasek]

Summary

• Fix OCI pusher failing to push images to OpenShift's internal registry when the repository doesn't exist yet
• OpenShift's registry returns HTTP 500 for manifest HEAD requests by digest on non-existent repos, which breaks remote.WriteIndex since it checks child manifests by digest
• Before writing the index, do a HEAD by tag to check repo existence; on 404, push an empty image to create the repo first, then proceed with the full index push

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.98%. Comparing base (a6ba689) to head (c20b65f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/oci/pusher.go	40.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3885      +/-   ##
==========================================
- Coverage   54.03%   53.98%   -0.05%     
==========================================
  Files         200      200              
  Lines       23567    23566       -1     
==========================================
- Hits        12734    12723      -11     
- Misses       9604     9612       +8     
- Partials     1229     1231       +2     

Flag	Coverage Δ
e2e	33.50% <40.00%> (+0.01%)	⬆️
e2e go	29.42% <50.00%> (+0.01%)	⬆️
e2e node	25.73% <0.00%> (+<0.01%)	⬆️
e2e python	29.76% <50.00%> (+0.01%)	⬆️
e2e quarkus	25.86% <0.00%> (+<0.01%)	⬆️
e2e rust	25.33% <0.00%> (+0.01%)	⬆️
e2e springboot	24.01% <0.00%> (+<0.01%)	⬆️
e2e typescript	25.85% <0.00%> (+<0.01%)	⬆️
e2e-config-ci	26.97% <50.00%> (+0.01%)	⬆️
integration	15.70% <0.00%> (-0.03%)	⬇️
unit macos-14	42.88% <50.00%> (-0.04%)	⬇️
unit macos-latest	42.88% <50.00%> (-0.04%)	⬇️
unit ubuntu-24.04-arm	43.20% <40.00%> (-0.04%)	⬇️
unit ubuntu-latest	43.74% <50.00%> (-0.04%)	⬇️
unit windows-latest	42.95% <50.00%> (-0.04%)	⬇️

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[gauron99]

/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gauron99, matejvasek

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• OWNERS [gauron99,matejvasek]

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[matejvasek]

/cherry-pick release-1.22

[matejvasek]

/cherry-pick release-1.21

[knative-prow-robot]

@matejvasek: new pull request created: #3886

Details

In response to this:

/cherry-pick release-1.22

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[knative-prow-robot]

@matejvasek: new pull request created: #3887

Details

In response to this:

/cherry-pick release-1.21

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.