proxy: Make OpenImageOptional work with oci-archive:

By adding the standard ENOENT to our list of errors.

I hit this while working on
coreos/rpm-ostree#4598
which is a tool that builds base images and wants to query
if the image exists beforehand.

cmd/skopeo/proxy.go

@@ -235,7 +235,8 @@ func isDockerManifestUnknownError(err error) bool {
 // TODO drive this into containers/image properly
 func isNotFoundImageError(err error) bool {
 	return isDockerManifestUnknownError(err) ||
-		errors.Is(err, ocilayout.ImageNotFoundError{})
+		errors.Is(err, ocilayout.ImageNotFoundError{}) ||
+		errors.Is(err, os.ErrNotExist)
 }
 
 func (h *proxyHandler) openImageImpl(args []any, allowNotFound bool) (retReplyBuf replyBuf, retErr error) {

integration/proxy_test.go

@@ -354,4 +354,11 @@ func (s *proxySuite) TestProxy() {
 		err = fmt.Errorf("Testing optional image %s: %v", knownNotExtantImage, err)
 	}
 	assert.NoError(t, err)
+
+	nonExistentArchive := "oci-archive:/enoent"
+	err = runTestOpenImageOptionalNotFound(p, nonExistentArchive)
+	if err != nil {
+		err = fmt.Errorf("Testing optional image %s: %v", nonExistentArchive, err)
+	}
+	assert.NoError(t, err)
 }