Use apt-cdrom ou configure /etc/apt/sources.list manualmente.
A página apt do debian secure indica que o apt pode ser configurado para sempre confiar nos arquivos de um CD. No entanto, não consigo encontrar documentação de como para fazer essa configuração.
It's also worth noting that newer versions of the Debian installer use the same signed Release file mechanism during their debootstrap of the Debian base system, before apt is available, and that the installer even uses this system to verify pieces of itself that it downloads from the net. Also, Debian does not currently sign the Release files on its CDs; apt can be configured to always trust packages from CDs so this is not a large problem.
Como eu configuro o apt para confiar em um CD?