Builder User
The builder user will execute build commands. 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 builder
Switch to the builder user and create a RSA key.
sudo su - builder
ssh-keygen -t rsa -C "builder@build-server" -qN ""
Add builder to AllowedUsers in the SSH config.
/etc/ssh/sshd_config
AllowedUsers admin builder
Put your local machine's public RSA key (usually in ~/.ssh/id_rsa.pub) in builder's authorized_keys.
echo "yourpublickey" >> ~/.ssh/authorized_keys
Switch back to the admin user.
exit