diskimage-builder/diskimage_builder/elements/ubuntu-minimal/pre-install.d/01-ubuntu-apt-update

#!/bin/bash
# Copyright (c) 2014 Hewlett-Packard Development Company, L.P.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.
#
# See the License for the specific language governing permissions and
# limitations under the License.

if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
    set -x
fi
set -eu
set -o pipefail

# We should manage this in a betterer way
cat << EOF >/etc/apt/sources.list
deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE ${DIB_DEBIAN_COMPONENTS//,/ }
deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE-updates ${DIB_DEBIAN_COMPONENTS//,/ }
deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE-backports ${DIB_DEBIAN_COMPONENTS//,/ }
deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE-security ${DIB_DEBIAN_COMPONENTS//,/ }
EOF

# Need to update to retrieve the signed Release file
apt-get update

apt-get clean
apt-get dist-upgrade -y