The installer is pretty much standard, but we need to add our own key to make the installer accept our mirror

APTOPTIONS="Apt::Architecture=i386
            Dir::Etc::sourcelist=$PWD/apt/sources.list
            Dir::State=$PWD/apt/state
            Dir::State::Status=$PWD/apt/status
            Dir::Cache=$PWD/apt/cache
            Dir::Cache::Archives=$PWD/incoming
            Debug::NoLocking=true"

apt-get $(for OPT in $APTOPTIONS ; do echo -n "-o $OPT " ; done) source debian-installer
INSTALLER=$(find . -maxdepth 1 -type d -name "debian-installer-*")
echo "deb file:$PWD etch main/debian-installer" >>  $INSTALLER/build/sources.list.udeb.local
    if [ -f $INSTALLER/build/config/local ] ; then
    grep -v "^KEYRING" $INSTALLER/build/config/local > $INSTALLER/build/config/local.new
    mv $INSTALLER/build/config/local.new $INSTALLER/build/config/local
fi
cat /usr/share/keyrings/debian-archive-keyring.gpg ~/.gnupg/pubring.gpg > $INSTALLER/build/config/local_keyrings.gpg
echo "KEYRING=config/local_keyrings.gpg" >> $INSTALLER/build/config/local
fakeroot make -C $INSTALLER/build/ build_netboot
rsync -av --delete  $INSTALLER/build/dest/netboot/ netboot/
rm -rf debian-installer*