Start with a clone of the ubuntu clean image.

Crete a user that will run the server proces:
useradd –comment “7days to die Server” -m 7days

change his shell:
chsh -s /bin/bash 7days

set his password:
passwd 7days

open the firewall for the ports of the server you decidet to run on:
firewall-cmd --permanent --add-port=26901/tcp
firewall-cmd --permanent --add-port=26901/udp
firewall-cmd --reload