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

#!/bin/bash
# Override the default /etc/apt/sources.list with $DIB_APT_SOURCES

set -eu
set -o pipefail

# exit directly if DIB_APT_SOURCES is not defined properly
if [ -z "$DIB_APT_SOURCES" ] ; then
    echo "DIB_APT_SOURCES must be set to the location of a sources.list file you wish to use"
    exit 1
elif [ ! -f "$DIB_APT_SOURCES" -o ! -s "$DIB_APT_SOURCES" ] ; then
    echo "$DIB_APT_SOURCES  is not a valid sources.list file."
    echo "You should assign proper sources.list file in DIB_APT_SOURCES"
    exit 1
fi

DIB_APT_SOURCES=`readlink -f $DIB_APT_SOURCES`

# copy the sources.list to cloudimg
pushd $TMP_MOUNT_PATH/etc/apt/
sudo cp -f $DIB_APT_SOURCES sources.list
popd