Create an SSH User with a non-interactive Shell

Create the Restricted System User

sudo useradd -r -s /bin/false -m -d /home/proxyuser proxyuser

Set Up Authentication

sudo passwd proxyuser

Configure SSH Daemon

sudo nano /etc/ssh/sshd_config

Match User proxyuser
    AllowAgentForwarding no
    AllowTcpForwarding yes
    X11Forwarding no
    PermitTTY no
    ForceCommand /bin/false

Restart the SSH Service

sudo systemctl restart ssh
# or
sudo systemctl restart sshd