Политика apt-cache показывает хранилище apt, которого нет в моих sources.list

В новом образе Dobu для Ubuntu 16.04 я запустил apt-get update и затем применил политику apt:

root@7b9b53f65af4:/etc/apt# apt-cache policy apt-file
apt-file:
  Installed: (none)
  Candidate: 2.5.5ubuntu1
  Version table:
     2.5.5ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages

Но я не вижу этого репозитория apt нигде в / etc / apt. Это ожидаемое?

root@7b9b53f65af4:/etc/apt# grep -ri 'amd64' .
root@7b9b53f65af4:/etc/apt#

Я нашел их, но они не совпадают:

root@7b9b53f65af4:/etc/apt# grep -ri 'xenial.universe' *
sources.list:deb http://archive.ubuntu.com/ubuntu/ xenial universe
sources.list:# deb-src http://archive.ubuntu.com/ubuntu/ xenial universe

Полное содержание /etc/apt/sources.list вставлено ниже. Других файлов .list в этом каталоге или его подкаталогах нет.

root@7b9b53f65af4:/etc/apt# cat sources.list
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu/ xenial main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ xenial main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu/ xenial-updates main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ xenial universe
# deb-src http://archive.ubuntu.com/ubuntu/ xenial universe
deb http://archive.ubuntu.com/ubuntu/ xenial-updates universe
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://archive.ubuntu.com/ubuntu/ xenial multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ xenial multiverse
deb http://archive.ubuntu.com/ubuntu/ xenial-updates multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu xenial partner
# deb-src http://archive.canonical.com/ubuntu xenial partner

deb http://security.ubuntu.com/ubuntu/ xenial-security main restricted
# deb-src http://security.ubuntu.com/ubuntu/ xenial-security main restricted
deb http://security.ubuntu.com/ubuntu/ xenial-security universe
# deb-src http://security.ubuntu.com/ubuntu/ xenial-security universe
deb http://security.ubuntu.com/ubuntu/ xenial-security multiverse
# deb-src http://security.ubuntu.com/ubuntu/ xenial-security multiverse