Use the result of reference.Parse when checking for digests.

No need to do a separate string search when reference.Parse has already
determined this.

Changes behavior: non-SHA256 digest formats are now recognized as well.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Closes: #1112
Approved by: rhatdan

libpod/image/parts.go

@@ -2,7 +2,6 @@ package image
 
 import (
 	"fmt"
-	"strings"
 
 	"github.com/containers/image/docker/reference"
 )
@@ -31,7 +30,7 @@ func decompose(input string) (imageParts, error) {
 	ntag, isTagged := imgRef.(reference.NamedTagged)
 	if !isTagged {
 		tag = "latest"
-		if strings.Contains(input, "@sha256:") {
+		if _, hasDigest := imgRef.(reference.Digested); hasDigest {
 			tag = "none"
 		}
 	} else {