docker: reference: load sha256 explicitly

There has been a change in upstream (docker/distribution), where they
now require users of the digest library to load algorithms into the
binary (in other words they do .Available checks on crypto algorithms).

This fixes the unit test failures with "unsupported digest algorithm".

Signed-off-by: Aleksa Sarai <asarai@suse.de>

docker/reference/reference.go

@@ -6,6 +6,9 @@ import (
 
 	"github.com/pkg/errors"
 
+	// "docker/distribution/digest" requires us to load the algorithms that we
+	// want to use into the binary (it calls .Available).
+	_ "crypto/sha256"
 	"github.com/docker/distribution/digest"
 	distreference "github.com/docker/distribution/reference"
 )
@@ -55,7 +58,7 @@ type Canonical interface {
 func ParseNamed(s string) (Named, error) {
 	named, err := distreference.ParseNamed(s)
 	if err != nil {
-		return nil, errors.Errorf("Error parsing reference: %q is not a valid repository/tag", s)
+		return nil, errors.Wrapf(err, "Error parsing reference: %q is not a valid repository/tag", s)
 	}
 	r, err := WithName(named.Name())
 	if err != nil {

docker/reference/reference_test.go

@@ -3,6 +3,7 @@ package reference
 import (
 	"testing"
 
+	_ "crypto/sha256"
 	"github.com/docker/distribution/digest"
 )