如何使用另一个容器中的golang Web应用程序连接到MONGODB容器

I have a golang web app that use mongodb. I create two containers: one for my_app and the other for mongodb. I use the official mongo-go-driver/mongo.

var client *mongo.Client

//create a mongo.Client
client, err = mongo.NewClient("mongodb://localhost:27017")
//client, err = mongo.NewClient("mongodb://my_first_mongodb:27017")
//client, err = mongo.NewClient("mongodb://:27017")
if err != nil {
    log.Printf("could not create a mongo client:  %v", err)
    panic(err)
}

//connect it to your running MongoDB server
ctx1, cancel1 := context.WithTimeout(context.Background(), 120*time.Second)
defer cancel1()

//Connect initializes the Client by starting background monitoring goroutines.
err = client.Connect(ctx1)
if err != nil {
    log.Printf("could not connect with mongodb:  %v", err)
    panic(err)
}

fmt.Println("trying to start mongodb for my app")
ctx2, cancel2 := context.WithTimeout(context.Background(), 120*time.Second)
defer cancel2()
err = client.Ping(ctx2, nil)
if err != nil {
    log.Printf("no ping to mongo db could not connect with mongodb:  %v", err)
    panic(err)
}

My problem is that I don't know how get(connect) the address ip (localhost) of mongodb inside a container. if I use

client, err = mongo.NewClient("mongodb://localhost:27017")

doesn't work. but if inside a docker-compose I set the container name (my_first_mongodb) and I use this code:

client, err = mongo.NewClient("mongodb://my_first_mongodb:27017")

is all fine and it is working.

The problem with this solution is that: Because Docker container names must be unique, you cannot scale a service beyond 1 container if you have specified a custom name. Attempting to do so results in an error here.

Any help please. Thanks