add dockerfile with multistage

cmd/app/main.go

@@ -1,9 +1,20 @@
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"log"
+
+	"github.com/spf13/viper"
+)
 
 func main() {
-	fmt.Println("Hello world")
+	viper.AddConfigPath("config")
+	err := viper.ReadInConfig()
+	if err != nil {
+		log.Fatalf("Error loading file: %v", err.Error())
+	}
+	api_key := viper.GetString("API_KEY")
+	fmt.Println("Hello world, and API_KEY:", api_key)
 	c := fmt.Sprintf("Test text: %s", "formatted")
 	fmt.Printf("Some text: %s", c)
 }