debian: Fix container creation on missing cache
debian: Fix container creation on missing cache

This is currently breaking our daily image builds which happen in a
perfectly clean environment without a Debian keyring and without
anything in /var/cache/lxc

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

ysbnim/lxc,cjwatson/lxc,federicobriata/lxc,ksperis/lxc,hallyn/lxc,Distrotech/lxc,ksperis/lxc,akshaykarle/lxc,bostjan/lxc,federicobriata/lxc,hfuCN/lxc,GreatFruitOmsk/lxc,caglar10ur/lxc,terceiro/lxc,thmo/lxc,Distrotech/lxc,cjwatson/lxc,LynxChaus/lxc,thmo/lxc,ss1h2a3tw/lxc,hallyn/lxc,caglar10ur/lxc,ksperis/lxc,hfuCN/lxc,ysbnim/lxc,tych0/lxc,QingweiPeterLan/cs188-lxc,LynxChaus/lxc,terceiro/lxc,GreatFruitOmsk/lxc,bostjan/lxc,terceiro/lxc,aeris/lxc,armcc/lxc,ss1h2a3tw/lxc,bostjan/lxc,hfuCN/lxc,tych0/lxc,caglar10ur/lxc,LynxChaus/lxc,Distrotech/lxc,QingweiPeterLan/cs188-lxc,QingweiPeterLan/cs188-lxc,ss1h2a3tw/lxc,aeris/lxc,aeris/lxc,cjwatson/lxc,tych0/lxc,hallyn/lxc,armcc/lxc,akshaykarle/lxc,thmo/lxc,armcc/lxc,GreatFruitOmsk/lxc,akshaykarle/lxc,federicobriata/lxc,ysbnim/lxc