LXC templates ship without a configured locale, so every apt/perl run
warned 'Setting locale failed'. setup_base_apt now exports C.UTF-8 for
the install run itself, installs the locales package, generates
en_US.UTF-8 and sets it as the system default via update-locale.
prompt_lxc_config asks 'SSH-Root-Login erlauben? [Y/n]' (env-presettable
via SSH_ROOT_LOGIN, validated, normalized to yes|no). The bootstrap passes
the value into the container; configure_ssh_root_login writes
/etc/ssh/sshd_config.d/zz-root-login.conf (yes -> PermitRootLogin yes,
no -> prohibit-password) and reloads sshd.