The RadDock has a fixed width, configured in pixels, so fitting its content in the ContentContainer of the control is not possible out of the box. Also, there is a small distance between the edge of the dock and its content.
Nevertheless, you could use custom script to configure the width of the dock according to its content and you can set it RenderMode to Lightweight in order to remove the space at the edge easily. Please check the attached sample and use it as a reference for achieving the desired result.
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