DEBIAN_FRONTEND=noninteractive apt-get ...
This is also applicable to
man 7 debconf:
This is the anti-frontend. It never interacts with you at all,
and makes the default answers be used for all questions. It
might mail error messages to root, but that's it; otherwise it
is completely silent and unobtrusive, a perfect frontend for
automatic installs. If you are using this front-end, and require
non-default answers to questions, you will need to preseed the
debconf database; see the section below on Unattended Package
Installation for more details.
If you do set
noninteractive, you should consider answering
debconf questions by using