/usr/share/gnome-session/sessions/cairo-dock.session is in cairo-dock-data 3.3.99.beta1.2.really.3.3.2-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | [GNOME Session]
Name=Cairo-Dock
RequiredComponents=gnome-settings-daemon;compiz;cairo-dock;
DesktopName=Unity
|