Prepare env

export LC_ALL=C
apt-get install --no-install-recommends cryptsetup debootstrap btrfs-progs

Prepare chroot

cryptsetup luksOpen /dev/sda2 root_crypt
mount /dev/mapper/root_crypt /mnt -o compress-force=lzo
mount /dev/sda1 /mnt/boot

Bind

mount --rbind /dev /mnt/dev
mount --rbind /sys /mnt/sys
mount --rbind /proc /mnt/proc

debootstrap

debootstrap --arch amd64 --include=cryptsetup,btrfs-progs,e2fsprogs,apt-utils,bash-completion bionic /mnt http://fr.archive.ubuntu.com/ubuntu/ 

Enter chroot

chroot /mnt/ /bin/bash

Install packages

apt install cryptsetup btrfs-progs e2fsprogs dropbear-initramfs openssh-server bash-completion
apt install linux-image-generic grub2 linux-firmware intel-microcode
apt install bind9-host dnsutils psmisc curl
apt install apt-transport-https

Unmount

umount /mnt/boot
umount -l /mnt/{sys,proc,dev}
umount /mnt/

Close

cryptsetup luksClose root_crypt
sync