disable schema1 by default, add a config flag to enable it

port of #2473

Signed-off-by: Viktor Stanchev <me@viktorstanchev.com>

registry/storage/v1unsupportedhandler.go

@@ -0,0 +1,23 @@
+package storage
+
+import (
+	"context"
+
+	"github.com/docker/distribution"
+	digest "github.com/opencontainers/go-digest"
+)
+
+// signedManifestHandler is a ManifestHandler that unmarshals v1 manifests but
+// refuses to Put v1 manifests
+type v1UnsupportedHandler struct {
+	innerHandler ManifestHandler
+}
+
+var _ ManifestHandler = &v1UnsupportedHandler{}
+
+func (v *v1UnsupportedHandler) Unmarshal(ctx context.Context, dgst digest.Digest, content []byte) (distribution.Manifest, error) {
+	return v.innerHandler.Unmarshal(ctx, dgst, content)
+}
+func (v *v1UnsupportedHandler) Put(ctx context.Context, manifest distribution.Manifest, skipDependencyVerification bool) (digest.Digest, error) {
+	return digest.Digest(""), distribution.ErrSchemaV1Unsupported
+}