This is how I got openvpn client work from startup:
yum install openvpn
systemctl enable openvpn@[conf_name].service
systemctl start openvpn@[conf_name].service
Then, make sure openvpn starts after Docker service.
nano /etc/systemd/system/multi-user.target.wants/openvpn@[conf_name].service
[Unit]
Description=OpenVPN Robust And Highly Flexible Tunneling Application On %I
Requires=docker.service
After=network.target docker.service
Then check if tun0 is up:
ifconfig