VPN Unlimited support (#499)

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

internal/updater/providers/vpnunlimited/string.go

@@ -0,0 +1,14 @@
+package vpnunlimited
+
+import "github.com/qdm12/gluetun/internal/models"
+
+func Stringify(servers []models.VPNUnlimitedServer) (s string) {
+	s = "func VPNUnlimitedServers() []models.VPNUnlimitedServer {\n"
+	s += "	return []models.VPNUnlimitedServer{\n"
+	for _, server := range servers {
+		s += "		" + server.String() + ",\n"
+	}
+	s += "	}\n"
+	s += "}"
+	return s
+}