This is actually for Designer, but I don't see it listed.

File->New

Create Main Window

Add Dock Widget via drag and drop

^Z to undo

-Notice that the dock widget is not present in the visual representation, but is still present in the Object Inspector.  Also note, that the dock cannot be delete and will throw an error :

"The container extension of the widget 'MainWindow' (QMainWindow) returned a widget not managed by Designer 'dockWidget' (QDockWidget) when queried for page #3. Container pages should only be added by specifying them in XML returned by the domXml() method of the custom widget."