debian: Add buster as a valid release
debian: Add buster as a valid release

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

thmo/lxc,hallyn/lxc,hallyn/lxc,caglar10ur/lxc,terceiro/lxc,caglar10ur/lxc,terceiro/lxc,caglar10ur/lxc,federicobriata/lxc,ss1h2a3tw/lxc,federicobriata/lxc,thmo/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,hallyn/lxc,federicobriata/lxc,thmo/lxc,terceiro/lxc