/usr/share/xsessions/cairo-dock.desktop 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 5 6 7 | [Desktop Entry]
Name=Cairo-Dock (GNOME)
Comment=This session logs you into GNOME with Cairo-Dock
Exec=gnome-session --session=cairo-dock
TryExec=cairo-dock-session
Icon=
Type=Application
|