Fix: assetPrefix refers to correct domains for production & preview environments (e2b-dev#570)

This pr fixes the following issue for
[e2b-dev#569](e2b-dev#569):

The production vercel environment did not refer assets to the correct
production domain through VERCEL_URL. Hence a custom environment
variable was added to the production environment to account for that.

Author: jakubno

apps/web/next.config.mjs

@@ -50,7 +50,9 @@ const nextConfig = {
   basePath: '',
   assetPrefix:
     process.env.NODE_ENV === 'production'
-      ? `https://${process.env.VERCEL_URL}`
+      ? `https://${
+          process.env.DASHBOARD_PROXY_DOMAIN ?? process.env.VERCEL_URL
+        }`
       : undefined,
   headers: async () => [
     {