How do I run gnome-shell in my custom LiveCD?

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http://askubuntu.com – I've made custom Ubuntu with Ubuntu builder. I've installed gnome-shell and cairo-dock from chroot. I've removed Unity from chroot too. I've edited /etc/lightdm/lightdm.conf, changed user-session = cairo-dock and then I built the custom ubuntu. I then ran it on virtual box. LiveCD successfully showed the Cairo dock but when I change to user-session = gnome-shell, desktop shows unity panel. I changed to gnome only and it still shows unity. Why Cairo dock works and not gnome-shell? (HowTos)