How can I use unicorn as "rails s"?

It looks like the unicorn-rails gem that @Dogbert mentioned can actually be used to make Unicorn the rails server handler.

Simply include gem "unicorn-rails" (and for Rails 4.2.4, gem "rack-handlers") in your Gemfile, run bundle install to install the gem, then you can run:

$ rails server unicorn

Although once unicorn-rails is installed, Unicorn should be the default app server so you could also just run rails server and it should use Unicorn (assuming you don't also have Thin or Mongrel in your Gemfile, in which case they may conflict and you might want to remove the ones you're not using).

A better option might just be to run the unicorn server directly.

bundle exec unicorn -p 3000 # default port is 8080

gem 'rack-handlers'

rails server unicorn