cache: add fallback for snapshotID

In older BuildKit versions snapshotID was not always set if record
was not created with GetByBlob method. Old code defaulted to cache
record ID in that case but that broke with the metadata interface
refactor.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit 2c0c429)

cache/metadata.go

@@ -251,7 +251,13 @@ func (md *cacheMetadata) queueMediaType(str string) error {
 }
 
 func (md *cacheMetadata) getSnapshotID() string {
-	return md.GetString(keySnapshot)
+	sid := md.GetString(keySnapshot)
+	// Note that historic buildkit releases did not always set the snapshot ID.
+	// Fallback to record ID is needed for old build cache compatibility.
+	if sid == "" {
+		return md.ID()
+	}
+	return sid
 }
 
 func (md *cacheMetadata) queueSnapshotID(str string) error {