fix: customize eai publicAssetURL in production

The `lib/startup`-addon is used to create a custom
index.html page to boot up the app. Because this is
a custom approach and we aren't relying on setting
rootURL to a different value when running the app
in `production` we need to set `publicAssetURL`
in the configuration we pass to `ember-auto-import`.

Reference:
9a2887f1d0/docs/upgrade-guide-2.0.md (quick-summary)

ui/packages/consul-ui/ember-cli-build.js

@@ -17,6 +17,7 @@ module.exports = function (defaults, $ = process.env) {
 
   $ = utils.env($);
   const env = EmberApp.env();
+  const isProd = ['production'].includes(env);
   const prodlike = ['production', 'staging'];
   const devlike = ['development', 'staging'];
   const sourcemaps = !['production'].includes(env) && !$('BABEL_DISABLE_SOURCEMAPS', false);
@@ -197,6 +198,7 @@ module.exports = function (defaults, $ = process.env) {
       autoImport: {
         // allows use of a CSP without 'unsafe-eval' directive
         forbidEval: true,
+        publicAssetURL: isProd ? '{{.ContentPath}}assets' : undefined,
       },
       codemirror: {
         keyMaps: ['sublime'],