fix(gluetun): add OpenVPN process user and cleanup stale config

- Add OPENVPN_PROCESS_USER=root, PUID=0, PGID=0 to default .env
  to prevent gluetun from injecting 'user' directive into target.ovpn
  which causes 'Unrecognized option' errors on Debian
- Add ExecStartPre to remove stale /etc/openvpn/target.ovpn before start
- Fixes #12988

install/gluetun-install.sh

@@ -46,6 +46,9 @@ VPN_TYPE=openvpn
 OPENVPN_CUSTOM_CONFIG=/opt/gluetun-data/custom.ovpn
 OPENVPN_USER=
 OPENVPN_PASSWORD=
+OPENVPN_PROCESS_USER=root
+PUID=0
+PGID=0
 HTTP_CONTROL_SERVER_ADDRESS=:8000
 HTTPPROXY=off
 SHADOWSOCKS=off
@@ -76,6 +79,7 @@ User=root
 WorkingDirectory=/opt/gluetun-data
 EnvironmentFile=/opt/gluetun-data/.env
 UnsetEnvironment=USER
+ExecStartPre=/bin/sh -c 'rm -f /etc/openvpn/target.ovpn'
 ExecStart=/usr/local/bin/gluetun
 Restart=on-failure
 RestartSec=5