Add an aside callout about dockerfile-rails gem

[heliocola]

Summary of changes

Add a notice about dockerfile-rails gem

Related Fly.io community and GitHub links

Update the Update section here: https://fly.io/docs/rails/getting-started/dockerfiles/#updates

Notes

If you have a Rails app that was created a while ago, you may not have dockerfile-rails gem in your Gemfile, so this command will not work! :-D