Fix sandbox command for Rails 8

Rails 8 does not create a sprockets manifest anymore,
but we still depend on it. Since rails commands do not
work anymore, because sprockets raises an error if no
manifest.js file is present we need to create it in
order to be able to run rails new

bin/sandbox

@@ -40,6 +40,13 @@ function unbundled {
 echo "~~~> Removing the old sandbox"
 rm -rf ./sandbox
 
+mkdir -p sandbox/app/assets/config
+cat <<MANIFEST > sandbox/app/assets/config/manifest.js
+//= link_tree ../images
+//= link_directory ../javascripts .js
+//= link_directory ../stylesheets .css
+MANIFEST
+
 echo "~~~> Creating a pristine Rails app"
 rails_version=`bundle exec ruby -e'require "rails"; puts Rails.version'`
 bundle exec rails _${rails_version}_ new sandbox \