Unable to Authenticate to Docker image via Golang

I am new to Docker and Golang. I am trying to run Golang server locally and connect it with my postgres docker image.Basically I want the DB to run as docker container with persitant volume

Error

PS C:\Users\usero\Desktop\Operation_GatiShakti\goBackEnd> go run .
mongodb://mongoadmin:password@localhost:27017/?tls=false
2025/08/04 22:08:16 POSTGRES_URI: postgres://myuser:mypassword@localhost:5432/mydatabase?sslmode=disable
2025/08/04 22:08:16 Failed to connect to PostgreSQL: failed to connect to `user=myuser database=mydatabase`: [::1]:5432 (localhost): failed SASL auth: FATAL: password authentication failed for user "myuser" (SQLSTATE 28P01)
exit status 1

I have created my docker-compose.yml file as -

services:
  postgres:
    image: postgres:latest
    container_name: my-postgres
    environment:
      POSTGRES_DB: mydatabase
      POSTGRES_USER: myuser
      POSTGRES_PASSWORD: mypassword
    volumes:
      - postgres_data:/var/lib/postgresql/data
    ports:
      - "5432:5432"
    restart: unless-stopped

volumes:
  postgres_data:

Inside my Golang code the .env file has

POSTGRES_URI=postgres://myuser:mypassword@localhost:5432/mydatabase?sslmode=disable

The main.go file:

func main() {

    var pgPool *pgxpool.Pool

    //======================================
    // ENVIRONMENT VARIABLE
    //======================================
    err := godotenv.Load(".env")
    if err != nil {
        log.Fatal("Error loading .env file")
    }

    //======================================
    // MongoDB connection
    //======================================
    client, err := mongoFunctions.ConnectMongoDB()
    if err != nil {
        log.Fatalf("Failed to connect to MongoDB: %v", err)
    }
    defer func() {
        if err = client.Disconnect(context.Background()); err != nil {
            log.Fatalf("Failed to disconnect from MongoDB: %v", err)
        }
    }()

    // Set global MongoDB collection handle
    MONGO_DB_UPSC := client.Database("UPSC")

    //======================================
    // PostgreSQL connection
    //======================================
    pgPool, err = connectPostgres()
    if err != nil {
        log.Fatalf("Failed to connect to PostgreSQL: %v", err)
    }
    defer pgPool.Close()

    smtpConfig := resetPassword.SMTPConfig{
        Host:     "smtp.hostinger.com",
        Port:     "587",
        Username: "[email protected]",
        Password: os.Getenv("SMTP_PASSWORD"),
        From:     "[email protected]",
    }

    resetService := resetPassword.NewService(pgPool, smtpConfig)

    // Setup logger
    logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
    slog.SetDefault(logger)

    // Setup Gin router
    router := gin.Default()
    router.Use(corsMiddleware)

    //-----------------------------------
    // LOGIN
    //-----------------------------------
    router.POST("/login", ratelimit.LoginMiddleware(), func(c *gin.Context) {
        authentication.LoginUser(c, pgPool)
    })

    router.GET("/refresh", func(c *gin.Context) {
        authentication.RefreshHandler(c.Writer, c.Request)
    })

    router.GET("/me", func(c *gin.Context) {
        userID, err := authentication.ValidateAccessToken(c.Request)
        if err != nil {
            c.JSON(http.StatusUnauthorized, gin.H{"error": "Unauthorized"})
            return
        }
        c.JSON(http.StatusOK, gin.H{"id": userID})
    })

    //-----------------------------------
    // PASSWORD RESET
    //-----------------------------------
    router.POST("/resetpassword", resetService.ResetPasswordHandler)

    //-----------------------------------
    // MONGO
    //-----------------------------------
    router.GET("/UPSC_History", func(c *gin.Context) {
        mongoFunctions.GetAllDocument(c, MONGO_DB_UPSC, "History")
    })

    //---------------------------------------
    //  POSTGRES
    //---------------------------------------
    router.GET("/pg-users", func(c *gin.Context) {
        registration.GetPostgresUsersHandler(c, pgPool)
    })
    router.POST("/pg-users", func(c *gin.Context) {
        registration.CreatePostgresUserHandler(c, pgPool)
    })

    router.POST("/pg-users/check", func(c *gin.Context) {
        registration.CheckUserExistsHandler(c, pgPool)
    })

    router.Run(":8080")
}

// ✅ CORS middleware
func corsMiddleware(c *gin.Context) {
    origin := c.Request.Header.Get("Origin")
    if origin != "" {
        c.Writer.Header().Set("Access-Control-Allow-Origin", origin)
        c.Writer.Header().Set("Access-Control-Allow-Credentials", "true")
        c.Writer.Header().Set("Access-Control-Allow-Headers", "Content-Type")
        c.Writer.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS")
    }
    if c.Request.Method == "OPTIONS" {
        c.AbortWithStatus(204)
        return
    }
    c.Next()
}

// ✅ PostgreSQL connection
func connectPostgres() (*pgxpool.Pool, error) {
    uri := os.Getenv("POSTGRES_URI")
    log.Println("POSTGRES_URI:", os.Getenv("POSTGRES_URI"))
    if uri == "" {
        log.Fatal("POSTGRES_URI environment variable is required")

    }

    ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
    defer cancel()

    pool, err := pgxpool.New(ctx, uri)
    if err != nil {
        return nil, err
    }

    if err := pool.Ping(ctx); err != nil {
        return nil, err
    }

    return pool, nil
}

I have tried removing image and volumes and restarting container, creating a new user as well. Tried Localhost, 127.0.0.1 and 0.0.0.0 as Ip.Postgres Docker image logs show it runs on 0.0.0.0

This fails even on typing correct password

psql -h localhost -p 5432 -U myuser

. But commands like - docker exec -it postgres_demo psql -U postgres work