Web User
The web user will run the web server (nginx) and the Phoenix application. It will be a system user with no password. As such, it will only be accessible remotely with a RSA key. Create it now.
sudo useradd --system --shell=/bin/bash --create-home --home=/var/web web
Switch to the web user and create an RSA key.
sudo su - web
ssh-keygen -t rsa -C "web@phoenix-server" -qN ""
Add web to AllowedUsers in the SSH config.
/etc/ssh/sshd_config
AllowedUsers admin web
Put your local machine's public RSA key (usually in ~/.ssh/id_rsa.pub) in builder's authorized_keys.
echo "yourlocalpublickey" >> ~/.ssh/authorized_keys