i don't believe it to merely be due to apt-cdrom not being called. Apt-cdrom is setting up the /etc/apt/sources.list, but apt-get update is ignoring those entries.
You can manually enter the chroot and run apt-cdrom -a add, and see that the /etc/apt/sources.list is unchanged. apt-get update still fails. I think it might be due to the cdrom repository not being signed (no Releases.gpg file)? Though APT::Authenticate::TrustCDROM is set to "yes"...
