Don't require a Docker-Content-Digest header when deleting images

Per #1010 , it seems
the header is not populated by AWS ECR.

We were actually computing the digest from the manifest bytes
already, so this is both more robust and simpler.

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

docker/docker_image_src.go

@@ -610,8 +610,11 @@ func deleteImage(ctx context.Context, sys *types.SystemContext, ref dockerRefere
 		return errors.Errorf("Failed to delete %v: %s (%v)", ref.ref, manifestBody, get.Status)
 	}
 
-	digest := get.Header.Get("Docker-Content-Digest")
-	deletePath := fmt.Sprintf(manifestPath, reference.Path(ref.ref), digest)
+	manifestDigest, err := manifest.Digest(manifestBody)
+	if err != nil {
+		return fmt.Errorf("computing manifest digest: %w", err)
+	}
+	deletePath := fmt.Sprintf(manifestPath, reference.Path(ref.ref), manifestDigest)
 
 	// When retrieving the digest from a registry >= 2.3 use the following header:
 	//   "Accept": "application/vnd.docker.distribution.manifest.v2+json"
@@ -629,11 +632,6 @@ func deleteImage(ctx context.Context, sys *types.SystemContext, ref dockerRefere
 		return errors.Errorf("Failed to delete %v: %s (%v)", deletePath, string(body), delete.Status)
 	}
 
-	manifestDigest, err := manifest.Digest(manifestBody)
-	if err != nil {
-		return err
-	}
-
 	for i := 0; ; i++ {
 		url := signatureStorageURL(c.signatureBase, manifestDigest, i)
 		missing, err := c.deleteOneSignature(url)