##/etc/systemd/system/arch32.service:
[Unit]
Description=32-bit chroot

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/bin/arch32 start
ExecStop=/usr/local/bin/arch32 stop

[Install]
WantedBy=multi-user.target

##/usr/local/bin/arch32:
#!/bin/bash

# Add '/var/run /var/lib/dbus' to the list to enable pulseaudio.
dirs=(/dev /dev/pts /dev/shm /tmp /home)

case $1 in
    start)
        for d in "${dirs[@]}"; do
            mount -o bind $d /opt/arch32$d
        done
        ;;
    stop)
        for (( i = ${#dirs[@]} - 1; i >= 0; i-- )); do
            umount "/opt/arch32${dirs[i]}"
        done
        umount /opt/arch32/{proc,sys}
        ;;
    *)
        echo "usage: $0 (start|stop)"
        exit 1
esac