Have a working RPi

Download and dd Raspbian Lite (Debian Stretch) on a microSD card
Plug the RPi with screen and keyboard (SSH is disabled by default)
Log into RPi and set it up (sudo raspi-config: Expand Filesystem, enable SSH (Interfacing Options), downgrade GPU memory (Advanced Options, Memory Split, 16), update it (sudo rpi-update, sudo apt-get update, sudo apt-get upgrade, sudo apt-get dist-upgrade), whatever you want…)
Reboot without screen and keyboard

From now on, use SSH to log into RPi. We are using user pi.

Log in RPi and install Docker

curl -sSL https://get.docker.com | sh ; sudo usermod -aG docker pi