This demos a snippet of JavaScript that'll delay a forms submission to the server until all files have finished uploading. I've also written an accompanying blog post about it.
I've also included a Carrierwave setup that worked for me (which also adds tags the remote uploads), along with a fairly rudimentary test.
Create the app and add the Cloudinary addon
heroku create && heroku addons:create cloudinary:starter
Adding the buildpacks for Heroku (So it installs JavaScript dependencies, then bundles)
heroku buildpacks:set heroku/ruby && heroku buildpacks:add --index 1 heroku/nodejs
Installing JavaScript dependencies (Via Yarn)
brew install yarn && yarn
Copy the example database and environment files
cp config/database.yml.example config/database.yml && cp .env.example .env
Setup the database
bundle exec rails db:setup
Getting the CLOUDINARY_URL
environmental variable (I'm assuming you've run the Heroku setup at this point)
heroku config -s | grep CLOUDINARY >> .env
Turning on the server
bundle exec rails s
bundle exec rails test