Disable concurrent when precompile the assets

Concurrency are raising failure during the precompile.
There is an issue on sprocket related to this issue
rails/sprockets#581.

Author: David Kang

src/api/config/initializers/assets.rb

@@ -9,3 +9,10 @@
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in app/assets folder are already added.
 Rails.application.config.assets.precompile += ['webui2/webui2.css', 'webui2/application.js', 'webui/obs_factory/application.css', 'webui2/cm2/*.js']
+
+# FIXME: concurrency are raising failure during the precompile.
+# For more information https://github.com/rails/sprockets/issues/581
+# and https://github.com/sass/sassc-ruby/pull/138
+Rails.application.config.assets.configure do |env|
+  env.export_concurrent = false
+end