diskimage-builder/diskimage_builder/elements/apt-conf/extra-data.d/99-override-default-apt-conf

#!/bin/bash
# Override the default /etc/apt/apt.conf with $DIB_APT_CONF

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

# exit directly if DIB_APT_CONF is not defined properly
if [ -z "${DIB_APT_CONF:-}" ] ; then
    echo "DIB_APT_CONF is not set - no apt.conf will be copied in"
    exit 0
elif [ ! -f "$DIB_APT_CONF" ] ; then
    echo "$DIB_APT_CONF is not a valid apt.conf file."
    echo "You should assign a proper apt.conf file in DIB_APT_CONF"
    exit 1
fi

# copy the apt.conf to cloudimg
sudo cp -L -f $DIB_APT_CONF $TMP_MOUNT_PATH/etc/apt/apt.conf