Run Rails application using unicorn in RubyMine

In the Run Menu choose edit configuration and define a configuration as per the screenshot below.

Here is a minimum functional unicorn-dev.rb

worker_processes 4
listen 3000, tcp_nopush: true
timeout 60
pid ~/tmp/unicorn.pid
stderr_path "~/tmp/unicorn.stderr.log"
stdout_path "~/tmp/unicorn.stdout.log"