frontend: Add 30-day caching for some static assets (js/css/...)

To improve loading time. The angular js files are served with a hash value
in the filename, which enables long caching without the problem when updating
the scripts.

frontend/nginx.conf

@@ -6,6 +6,10 @@ server {
   location / {
     try_files $uri $uri/ /index.html;
   }
+  location ~* \.(js|css|png|jpg|jpeg|gif|svg|ico)$ {
+    expires 30d;
+    add_header Cache-Control "public, no-transform";
+  }
   location /api/ {
     proxy_pass ${API_BACKEND_URL};
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;