DEV: support -p in `rails s`

if you need to specify port

bin/rails

@@ -1,7 +1,12 @@
 #!/usr/bin/env ruby
 
 if !ENV["RAILS_ENV"] && (ARGV[0] == "s" || ARGV[0] == "server")
-  ENV["UNICORN_PORT"] = "3000"
+  ENV["UNICORN_PORT"] ||= "3000"
+
+  if ARGV[1] == "-p" && (port = ARGV[2].to_i) > 0
+    ENV["UNICORN_PORT"] = port.to_s
+  end
+
   ENV["RAILS_LOGS_STDOUT"] ||= "1"
   exec File.expand_path("unicorn", __dir__)
 end