VPN Unlimited support (#499)

- Fixes #420 
- Revert to docker/build-push-action@v2.4.0

internal/provider/vpnunlimited/connection.go

@@ -0,0 +1,44 @@
+package vpnunlimited
+
+import (
+	"errors"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+)
+
+var ErrProtocolUnsupported = errors.New("network protocol is not supported")
+
+func (p *Provider) GetOpenVPNConnection(selection configuration.ServerSelection) (
+	connection models.OpenVPNConnection, err error) {
+	const port = 1194
+	const protocol = constants.UDP
+	if selection.TCP {
+		return connection, ErrProtocolUnsupported
+	}
+
+	servers, err := p.filterServers(selection)
+	if err != nil {
+		return connection, err
+	}
+
+	var connections []models.OpenVPNConnection
+	for _, server := range servers {
+		for _, IP := range server.IPs {
+			connection := models.OpenVPNConnection{
+				IP:       IP,
+				Port:     port,
+				Protocol: protocol,
+			}
+			connections = append(connections, connection)
+		}
+	}
+
+	if selection.TargetIP != nil {
+		return utils.GetTargetIPConnection(connections, selection.TargetIP)
+	}
+
+	return utils.PickRandomConnection(connections, p.randSource), nil
+}