rails s 時のデフォルトのポート番号を変更する

#はじめに
複数のrailsアプリを立ち上げる時はポート番号が被らないようにrails s -p 4000のようにサーバーを立てた時にポート番号を指定するが、このアプリはデフォルトで3500番で起動したいなぁって時にどうやるのかを書いてみます。

#環境
rails: 5.2.2

#手順
railsアプリ内のconfig/puma.rbの12行目を変更する。

config/puma.rb

# Puma can serve each request in a thread from an internal thread pool.
# The `threads` method setting takes two numbers: a minimum and maximum.
# Any libraries that use thread pools should be configured to match
# the maximum value specified for Puma. Default is set to 5 threads for minimum
# and maximum; this matches the default thread size of Active Record.
#
threads_count = ENV.fetch("RAILS_MAX_THREADS") { 5 }
threads threads_count, threads_count

# Specifies the `port` that Puma will listen on to receive requests; default is 3000.
#
port        ENV.fetch("PORT") { 3000 } #ここに好きなポート番号を指定する

# Specifies the `environment` that Puma will run in.
#
environment ENV.fetch("RAILS_ENV") { "development" }

# Specifies the number of `workers` to boot in clustered mode.
# Workers are forked webserver processes. If using threads and workers together
# the concurrency of the application would be max `threads` * `workers`.
# Workers do not work on JRuby or Windows (both of which do not support
# processes).
#
# workers ENV.fetch("WEB_CONCURRENCY") { 2 }

# Use the `preload_app!` method when specifying a `workers` number.
# This directive tells Puma to first boot the application and load code
# before forking the application. This takes advantage of Copy On Write
# process behavior so workers use less memory.
#
# preload_app!

# Allow puma to be restarted by `rails restart` command.
plugin :tmp_restart

ここで3500と指定するとrails sだけでもポート番号:3500で起動してくれる。