Set default UserId and GroupId to 0:0

Update to match Dockerfile specifications when using ADD or COPY
Previous functionality was to preserve the user and group from the source, which may not exist in the container.
Fixes GoogleContainerTools#1921

pkg/util/command_util.go

@@ -354,7 +354,7 @@ Loop:
 
 func GetUserGroup(chownStr string, env []string) (int64, int64, error) {
 	if chownStr == "" {
-		return DoNotChangeUID, DoNotChangeGID, nil
+		return 0, 0, nil
 	}
 
 	chown, err := ResolveEnvironmentReplacement(chownStr, env, false)

pkg/util/command_util_test.go

@@ -567,8 +567,8 @@ func TestGetUserGroup(t *testing.T) {
 			mockIDGetter: func(string, string) (uint32, uint32, error) {
 				return 0, 0, fmt.Errorf("should not be called")
 			},
-			expectedU: -1,
-			expectedG: -1,
+			expectedU: 0,
+			expectedG: 0,
 		},
 	}
 	for _, tc := range tests {