Downloading the Stage Tarball:

mkdir /var/lib/machines/gentoo-stage3-amd64 && cd /var/lib/machines/gentoo-stage3-amd64
wget https://gentoo.osuosl.org/releases/amd64/autobuilds/current-stage3-amd64-systemd/stage3-amd64-systemd-*.tar.xz
tar xpvf stage3-*.tar.xz --xattrs-include='*.*' --numeric-owner

Config file: gentoo-stage3-amd64.nspawn

[Exec]
Boot=1
PrivateUsers=off

[Files]
BindReadOnly=/var/db/repos
BindReadOnly=/usr/src

Bind=/var/cache/distfiles
Bind=/var/cache/binpkgs
Bind=/var/tmp/portage

Set password:

systemd-nspawn --settings=no -D /var/lib/machines/gentoo-stage3-amd64
passwd
exit

Boot and update:

systemd-nspawn --machine=gentoo-stage3-amd64
emerge --metadata
emerge --deep --newuse --update --ask --jobs 4 @system
emerge bash-completition
systemctl poweroff

chroot

mount --types proc /proc /mnt/gentoo/proc
mount --rbind /sys /mnt/gentoo/sys
mount --make-rslave /mnt/gentoo/sys
mount --rbind /dev /mnt/gentoo/dev
mount --make-rslave /mnt/gentoo/dev 
chroot /mnt/gentoo /bin/bash
env-update
source /etc/profile
export PS1="(chroot) $PS1"

to install/update grub: mount /boot in chroot!

flushing the environment during chroot

env -i HOME=$HOME TERM=$TERM chroot /mnt/gentoo /bin/bash