Assuming that all local FS are alaready mounted in "/chroot/".

sh-4.1# mount -o bind /dev /chroot/dev/
sh-4.1# mount -o bind /sys /chroot/sys/
sh-4.1# mount -t proc /proc/ /chroot/proc/
sh-4.1# chroot /chroot/
sh-4.1# grub-install /dev/sda && reboot