Don’t overwrite layer checksum on push

After v1.8.3 layer checksum is used for image ID
validation. Rewriting the checksums on push would
mean that next pulls will get different image IDs
and pulls may fail if its detected that same
manifest digest can now point to new image ID.

Fixes #17178

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

graph/push_v2.go

@@ -169,15 +169,17 @@ func (p *v2Pusher) pushV2Tag(tag string) error {
 		// if digest was empty or not saved, or if blob does not exist on the remote repository,
 		// then fetch it.
 		if !exists {
-			if pushDigest, err := p.pushV2Image(p.repo.Blobs(context.Background()), layer); err != nil {
+			var pushDigest digest.Digest
+			if pushDigest, err = p.pushV2Image(p.repo.Blobs(context.Background()), layer); err != nil {
 				return err
-			} else if pushDigest != dgst {
+			}
+			if dgst == "" {
 				// Cache new checksum
 				if err := p.graph.SetLayerDigest(layer.ID, pushDigest); err != nil {
 					return err
 				}
-				dgst = pushDigest
 			}
+			dgst = pushDigest
 		}
 
 		// read v1Compatibility config, generate new if needed