Bootstrap Debian

upload debian-10.9.0-amd64-netinst.iso
boot from iso
install:      (text based)
lang:         english
country:      switzerland
locale:       US (en_US.UTF-8)
keymap:       Swiss German
nic:          ens3
hostname:     template-25G
domain:       your.domain.de
passwd:       xxxxxxxx
user:         firstname lastname / loginame
passwd:       xxxxxxxx
disk:         Guided - entire disk with LVM - (one partition | separate /home | separate /home, /var and /tmp)
              separate partition for large disks
              one partition for smaller disks
write:        yes
disk:         20GB (for guided partitioning), 5GB for Spare
write:        yes
another dvd:  no
mirror:       switzerland, debian.ethz.ch
survey:       no
software:     SSH Server, standard system utilities
grub:         yes, /dev/sda3

remove iso and reboot

login as user, su to root

mkdir /root/.ssh && chmod 600 /root/.ssh
echo "ssh-ed25519 AAAAxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" >> /root/.ssh/authorized_keys
chmod 600 /root/.ssh/authorized_keys

halt -p
-> snapshot template-debian-25G