feat(express-app): add environment configuration

.env.template

+PORT=
\ No newline at end of file

src/infrastructure/express-application.ts

 import { ExpressServer } from './express-server';
+import * as dotenv from 'dotenv';
 
 export class ExpressApplication {
-    private server = new ExpressServer('3000');
+    private server: ExpressServer;
+
+    constructor() {
+        this.configureEnvironment();
+
+        const port = this.getPort();
+        this.server = new ExpressServer(port);
+    }
 
     bootstrap(): void {
         this.server.bootstrap();
     }
+
+    private configureEnvironment(): void {
+        dotenv.config({
+            path: '.env',
+        });
+    }
+
+    private getPort(): string {
+        const port = process.env.PORT;
+        if (!port) {
+            throw new Error('No port was found in env file.');
+        }
+
+        return port;
+    }
 }