docker | database connection refused between two containers

I have a mysql database running with the following docker-compose.yml:

version: '3.3'    services:        db:            image: mysql:5.7            restart: always            environment:                MYSQL_DATABASE: 'demo'                # So you don't have to use root, but you can if you like                MYSQL_USER: 'user'                # You can use whatever password you like                MYSQL_PASSWORD: 'password'                # Password for root access                MYSQL_ROOT_PASSWORD: 'password'            ports:            # <Port exposed> : < MySQL Port running inside container>                - '3306:3306'            expose:                # Opens port 3306 on the container                - '3306'            # Where our data will be persisted            volumes:                - my-db:/var/lib/mysql            networks:                - backendnetworks:    backend:        driver: bridge# Names our volumevolumes:    my-db:

$ docker-compose build

$ docker-compose up

I have a basic golang rest api with the following Dockerfile:

# Start from golang:1.12-alpine base imageFROM golang:1.12-alpine# Adding git, bash and openssh to the imageRUN apk update && apk upgrade && \    apk add --no-cache bash git openssh# Set the Current Working Directory inside the containerWORKDIR /app# Copy the source from the current directory to the Working Directory inside the containerCOPY . .RUN go get -d github.com/gorilla/muxRUN go get -d github.com/go-sql-driver/mysqlRUN go get -d github.com/golang-migrate/migrateRUN go get -d github.com/golang-migrate/migrate/database/mysqlRUN go get -d github.com/golang-migrate/migrate/source/file# Build the Go appRUN go build -o main .# Expose port 8080 to the outside worldEXPOSE 8080# Run the executableCMD ["./main"]

and the following function is called:

func CreateDatabase() (*sql.DB, error) {    serverName := "localhost:3306"    user := "user"    password := "password"    dbName := "demo"    connectionString := fmt.Sprintf("%s:%s@tcp(%s)/%s?charset=utf8mb4&collation=utf8mb4_unicode_ci&parseTime=true&multiStatements=true", user, password, serverName, dbName)    db, err := sql.Open("mysql", connectionString)    if err != nil {        return nil, err    }    if err := migrateDatabase(db); err != nil {        return db, err    }    return db, nil}

$ docker run -p 80:8080 --network=<appname>-mysql_backend <imageid>

$ Database connection failed: %sdial tcp 127.0.0.1:3306: connect: connection refused

I cannot get the api to establish a database connection with the database container?