Unmount directories before spawning systemd-nspawn

gentoostrap.sh

@@ -197,6 +197,9 @@ build-gentoo() {
 	popd
 	log "Pivoting to container for more setup"
 	cp "$0" "$_optdest/gentoostrap.sh"
+	for dir in proc sys dev; do
+		umount "$_optdest/$dir" || :
+	done
 	sudo systemd-nspawn --directory="$_optdest" /gentoostrap.sh -c
 
 	# The nspawn container sadly cannot manipulate the environment entirely, so we need to now do an ACTUAL chroot to install GRUB