use variable for api base url

frontend/app/src/App.js

@@ -3,11 +3,13 @@ import {useEffect, useState} from "react";
 import axios from "axios";
 import PostList from "./components/Post/PostList";
 
+const apiUrl = "http://localhost:8000";
+
 function App() {
     const [postList, setPostsList] = useState([]);
 
     const fetchPosts = async () => {
-        await axios.get("http://localhost:8000/catalog")
+        await axios.get(apiUrl + "/catalog")
         .then((response) => { setPostsList(response.data)})
     }