diskimage-builder/diskimage_builder/elements/gentoo/pre-install.d/02-gentoo-03-enable-overlays

#!/bin/bash

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

if [[ ${GENTOO_OVERLAYS} != '' ]]; then
    if mountpoint -q /dev/shm; then
    echo "/dev/shm found in /proc/self/mountinfo"
    elif [[ -k /dev/shm ]]; then
        echo "/dev/shm exists and is stickied"
    else
        fix_shm
    fi

    if [[ ! -f /usr/portage/profiles ]]; then
        emerge-webrsync -q
    fi

    # layman requires cryptography, which needs ecc crypto functions.
    # binaries providing those functions have questionable legality for
    # redistribution, so we have to use a version of openssl that works around
    # it (using fedora's patchset) and also use a version of cryptography that
    # depends on that version of openssl.
    echo '=dev-python/cryptography-2.1.4-r2 ~amd64' >> /etc/portage/package.keywords/layman
    echo '=dev-libs/openssl-1.0.2o-r6 ~amd64' >> /etc/portage/package.keywords/layman
    emerge ${GENTOO_EMERGE_DEFAULT_OPTS} --oneshot openssl openssh
    # install layman
    emerge ${GENTOO_EMERGE_DEFAULT_OPTS} --oneshot --deep --ignore-built-slot-operator-deps=y layman
    # sync the initial overlay list
    layman -S
    # enable the various overlays, ignore failures (overlay my already be enabled)
    set +e
    for OVERLAY in ${GENTOO_OVERLAYS}; do
        layman -a "${OVERLAY}"
    done
    set -e

    unfix_shm
fi