SOURCE_DATE_EPOCH: containerimage: CreatedAt timestamp should be reproducible

[AkihiroSuda]

   And this timestamp is also non-reproducible

buildkit/exporter/containerimage/export.go

Lines 246 to 249 in 092e977

	img := images.Image{
	Target: *desc,
	CreatedAt: time.Now(),
	}

Originally posted by @AkihiroSuda in #2918 (comment)

[tonistiigi]

Isn't this the "pull time" that is not actually part of the image?

[AkihiroSuda]

Isn't this the "pull time" that is not actually part of the image?

Not actually part of the image, but I’d expect this to be set to the SOURCE_DATE_EPOCH.
Not a hard requirement.

[AkihiroSuda]

Looks like containerd does not support setting custom CreatedAt

https://github.com/containerd/containerd/blob/v1.7.0-beta.0/metadata/images.go#L147

https://github.com/containerd/containerd/blob/v1.7.0-beta.0/metadata/images.go#L183
https://github.com/containerd/containerd/blob/v1.7.0-beta.0/metadata/images.go#L230