Fresh container based on Debian 7 template does not have any dialog-like package installed (dialog, whiptail) which makes it impossible to properly configure any package inside container, which requires interactive configuration during installation.
Example of an error produced by the installation of such packages :
debconf: unable to initialize frontend: Dialog debconf: (No usable dialog-like program is installed, so the dialog based frontend cannot be used. at /usr/share/perl5/Debconf/FrontEnd/Dialog.pm line 76, <> line 3.) debconf: falling back to frontend: Readline
The problem is recognized as a product issue PSBM-22083 and will be fixed in one of the future product updates
Add the necessary package manually into the OS template :
~# echo whiptail >> /vz/template/debian/7.0/x86_64/config/os/default/packages_1
Recreate OS template cache :
~# vzpkg remove cache debian-7.0-x86_64 ~# vzpkg create cache debian-7.0-x86_64