Sign() should receive the digest from Push()

[AkihiroSuda]

nerdctl/pkg/cmd/image/push.go

Line 151 in 8a1f227

if err = signutil.Sign(rawRef, options.GOptions.Experimental, options.SignOptions); err != nil {

not related to this PR. Just notice when pushing an image, sign (line 152) happens after push (line 133-150). Is this expected?

IIUC signing after pushing is ok (for cosign, at least), but the current implementation is wrong anyway; the Sign() function should receive the digest from the Push() function to prohibit TOCTOU.

(I'm mentioning this bug publicly because the cosign integration is still experimental for nerdctl)

Originally posted by @AkihiroSuda in #2132 (comment)

[AkihiroSuda]

cc @developer-guy

[developer-guy]

yes, you are right we should rely on digests, not on the tags. I'll fix this