add Dockerfile to build the go app

golang/Makefile

@@ -1,6 +1,6 @@
 serv:
-	DB_DRIVER="postgres" DB_URL="user=crow password=4LlKpnQ2RZPzL13BSpkW4k dbname=crowsnest host=192.168.0.2 port=5432 sslmode=disable" go run cmd/frontend/*
+	DB_USER=crow DB_PASS=4LlKpnQ2RZPzL13BSpkW4k DB_NAME=crowsnest DB_HOST=10.99.0.3 go run cmd/frontend/*
 crawl:
-	DB_DRIVER="postgres" DB_URL="user=crow password=4LlKpnQ2RZPzL13BSpkW4k dbname=crowsnest host=192.168.0.2 port=5432 sslmode=disable" go run cmd/crawler/main.go
+	DB_USER=crow DB_PASS=4LlKpnQ2RZPzL13BSpkW4k DB_NAME=crowsnest DB_HOST=192.168.0.2 go run cmd/crawler/main.go
 summ:
-	DB_DRIVER="postgres" DB_URL="user=crow password=4LlKpnQ2RZPzL13BSpkW4k dbname=crowsnest host=192.168.0.2 port=5432 sslmode=disable" go run cmd/summarizer/main.go
+	DB_USER=crow DB_PASS=4LlKpnQ2RZPzL13BSpkW4k DB_NAME=crowsnest DB_HOST=192.168.0.2 go run cmd/summarizer/main.go

golang/cmd/frontend/main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"crowsnest/internal/model/database"
 	"database/sql"
+	"fmt"
 	"log"
 	"net/http"
 	"os"
@@ -16,14 +17,35 @@ type App struct {
 
 func main() {
 	// collect environement variables
-	databaseURL := os.Getenv("DB_URL")
+	dbPass := os.Getenv("DB_PASS")
+	if dbPass == "" {
+		log.Fatal("empty DB_PASS")
+	}
+	dbHost := os.Getenv("DB_HOST")
+	if dbHost == "" {
+		log.Fatal("empty DB_HOST")
+	}
+	dbPort := os.Getenv("DB_PORT")
+	if dbPort == "" {
+		dbPort = "5432"
+	}
+	dbUser := os.Getenv("DB_USER")
+	if dbUser == "" {
+		dbUser = "postgres"
+	}
+	dbName := os.Getenv("DB_NAME")
 
 	// connect to database
+	databaseURL := fmt.Sprintf("user=%s password=%s dbname=%s host=%s port=%s sslmode=disable",
+		dbUser, dbPass, dbName, dbHost, dbPort)
 	db, err := sql.Open("postgres", databaseURL)
 	if err != nil {
 		log.Fatal(err)
 	}
 	defer db.Close()
+	if err = db.Ping(); err != nil {
+		log.Fatal(err)
+	}
 
 	// define app
 	app := &App{