Add debug output to manifest list parsing

Per request for more debug info on how the engine deals with
multi-platform "manifest list" images, this adds information about the
manifest list entries and whether it found an os/arch match, and the
digest of the match.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

distribution/pull_v2.go

@@ -658,6 +658,7 @@ func (p *v2Puller) pullManifestList(ctx context.Context, ref reference.Named, mf
 		return "", "", err
 	}
 
+	logrus.Debugf("%s resolved to a manifestList object with %d entries; looking for a os/arch match", ref, len(mfstList.Manifests))
 	var manifestDigest digest.Digest
 	for _, manifestDescriptor := range mfstList.Manifests {
 		// TODO(aaronl): The manifest list spec supports optional
@@ -665,12 +666,15 @@ func (p *v2Puller) pullManifestList(ctx context.Context, ref reference.Named, mf
 		// Once they are, their values should be interpreted here.
 		if manifestDescriptor.Platform.Architecture == runtime.GOARCH && manifestDescriptor.Platform.OS == runtime.GOOS {
 			manifestDigest = manifestDescriptor.Digest
+			logrus.Debugf("found match for %s/%s with media type %s, digest %s", runtime.GOOS, runtime.GOARCH, manifestDescriptor.MediaType, manifestDigest.String())
 			break
 		}
 	}
 
 	if manifestDigest == "" {
-		return "", "", errors.New("no supported platform found in manifest list")
+		errMsg := fmt.Sprintf("no matching manifest for %s/%s in the manifest list entries", runtime.GOOS, runtime.GOARCH)
+		logrus.Debugf(errMsg)
+		return "", "", errors.New(errMsg)
 	}
 
 	manSvc, err := p.repo.Manifests(ctx)