What you are looking for is a new session that uses the dock alone. I believe then what you need is
cairo-dock. It will install a new session that you can select from your login screen, which allows you to use a dock-only session (so the Unity launcher and panel will not show up there).
When you login to the computer, there will be a button with the Ubuntu logo next to your name (don't enter your password yet).
Click this button and you will get a list of "sessions" that you can log in to. Choose Cairo-dock, and then enter your password and hit enter. You will not login to a Cairo-Dock session from now on.