feat: implement kaniko.imagePullSecret for pulling images from private registry w/ auth

[alphanota]

Original PR #9191
Fixes: #9170
Related: #9183 (closed), #9190
Merge before/after: after #9190

Description

Implements the config option, see #9183 and #9170

User facing changes (remove if N/A)
n/a
Follow-up Work (remove if N/A)
n/a

[code-review-assist]

Hi @alphanota, reviewers, and future readers!

I'm Gemini, and I'm currently reviewing this pull request. In the meantime, here's a quick summary to help everyone get up to speed:

This pull request, feat: implement kaniko.imagePullSecret for pulling images from private registry w/ auth, aims to add support for pulling Kaniko images from private registries using authentication. This addresses issue #9170 and builds upon the work in PRs #9183 and #9190.

Here's a breakdown of the changes:

• pkg/skaffold/build/cluster/pod.go (lines 95-103): This commit adds a new section to the kanikoPodSpec function. It checks for the presence of artifact.ImagePullSecret. If it's not empty, it adds a Kubernetes LocalObjectReference to the pod's Spec.ImagePullSecrets, allowing Kaniko to pull images from a private registry using the specified secret.
• pkg/skaffold/build/cluster/pod_test.go: Extensive test updates to cover the new functionality. The changes are too numerous to list here but ensure that the new feature functions correctly.
• pkg/skaffold/schema/latest/config.go (lines 1465-1473): A new field, ImagePullSecret, has been added to the KanikoArtifact struct. This allows users to specify the name of a Kubernetes secret containing the authentication credentials needed to pull Kaniko images from a private registry. This is a user-facing change.

I'll provide a more detailed review shortly.

---

And now, a little haiku to lighten the mood:

Secrets now revealed,
Private images, freely flow,
Builds run smoothly on.

[code-review-assist]

This PR adds support for imagePullSecret for Kaniko builds, addressing the need to pull base images from private registries. The implementation looks correct and the changes are well-localized. A few minor suggestions for improvement follow.