diskimage-builder/diskimage_builder/elements/ubuntu/pre-install.d/01-set-ubuntu-mirror

#!/bin/bash

if [ ${DIB_DEBUG_TRACE:-0} -gt 1 ]; then
    set -x
fi
set -eu
set -o pipefail

DIB_DISTRIBUTION_MIRROR=${DIB_DISTRIBUTION_MIRROR:-}

[ -n "$DIB_DISTRIBUTION_MIRROR" ] || exit 0

while IFS= read line
do
    if [[ "$line" =~ ${DIB_DISTRIBUTION_MIRROR_UBUNTU_IGNORE:-} ]]; then
        # append line unmodified
        echo "$line" | tee --append /etc/apt/sources.list.new
    else
        echo "$line" | \
            sed -e "s&http://\(archive\|security\).ubuntu.com/ubuntu&$DIB_DISTRIBUTION_MIRROR&" | \
            tee --append /etc/apt/sources.list.new
    fi
done < /etc/apt/sources.list

if [[ -n "${DIB_DISTRIBUTION_MIRROR_UBUNTU_INSECURE:-}" ]]; then
    echo "APT::Get::AllowUnauthenticated \"true\";" | tee /etc/apt/apt.conf.d/95allow-unauthenticated
    echo "Acquire::AllowInsecureRepositories \"true\";" | tee -a /etc/apt/apt.conf.d/95allow-unauthenticated
fi

mv /etc/apt/sources.list.new /etc/apt/sources.list