fix: sourceImage's CreatedAt timestamp should not be included in cache key

[mzihlmann]

#3088 (comment)

doesn't fix #3088 but is related to that issue

Description

Currently the CreatedAt timestamp (and all other metadata) is included into kaniko's cache key, this means that if a new image is created with the exact same layers, it invalidates caches in all builds that use it as a base.

Submitter Checklist

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• Includes unit tests
• Adds integration tests if needed.

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

• kaniko cache learned to ignore CreatedAt field in base images

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[mzihlmann]

labels can be excluded as well with this diff, which would indeed fix #3088

+ cf, err := sourceImage.ConfigFile()
+ if err != nil {
+ 	return nil, err
+ }
+ cfg := cf.DeepCopy()
+ cfg.Created = v1.Time{}
+ cfg.Config.Labels = map[string]string{}
+ sourceImageReproducible, err := mutate.ConfigFile(sourceImage, cfg)
+ if err != nil {
+ 	return nil, err
+ }
+ 
+ digest, err := sourceImageReproducible.Digest()
- digest, err := sourceImage.Digest()