Add UnavailableErrorCode (#1230)

For registries that return a structured error containing UNAVAILABLE, we
weren't retrying because this error code isn't define in any specs;
however, it is used in the wild!

pkg/v1/remote/transport/error.go

@@ -155,13 +155,18 @@ const (
 	UnsupportedErrorCode         ErrorCode = "UNSUPPORTED"
 	TooManyRequestsErrorCode     ErrorCode = "TOOMANYREQUESTS"
 	UnknownErrorCode             ErrorCode = "UNKNOWN"
+
+	// This isn't defined by either docker or OCI spec, but is defined by docker/distribution:
+	// https://github.com/distribution/distribution/blob/6a977a5a754baa213041443f841705888107362a/registry/api/errcode/register.go#L60
+	UnavailableErrorCode ErrorCode = "UNAVAILABLE"
 )
 
 // TODO: Include other error types.
 var temporaryErrorCodes = map[ErrorCode]struct{}{
 	BlobUploadInvalidErrorCode: {},
 	TooManyRequestsErrorCode:   {},
 	UnknownErrorCode:           {},
+	UnavailableErrorCode:       {},
 }
 
 var temporaryStatusCodes = map[int]struct{}{

pkg/v1/remote/transport/error_test.go

@@ -57,6 +57,13 @@ func TestTemporary(t *testing.T) {
 			}},
 		},
 		retry: true,
+	}, {
+		error: &Error{
+			Errors: []Diagnostic{{
+				Code: UnavailableErrorCode,
+			}},
+		},
+		retry: true,
 	}, {
 		error: &Error{
 			StatusCode: http.StatusInternalServerError,