heroku-buildpack-ruby fetches bundler, yaml and ruby (twice!) from S3 every time you deploy. This is slow and it makes me sad to wait. Luckily it allows a BUILDPACK_VENDOR_URL. This project can be your vendor url.
Clone and deploy this project somewhere then in your own project:
dokku config:set my-app --no-restart BUILDPACK_VENDOR_URL=https://url.to.this.project/heroku-buildpack-rubyThat's it! Now just deploy like you would normally.
First time the vendor is asked for a tgz it fetches it from S3 and 1) sends it back to the deployment right away but also 2) saves a copy to /tmp.
Next time it's asked, it sends directly from the cached copy.
MIT