Allow using env var to specify pidfile

[benthorner]

Summary

Previously it was only possible to specify the location of the pidfile
for the 'rails server' command with the '-P' flag. This adds support for
specifying the pidfile using a PIDFILE env var, which can still be
overridden by the '-P' flag and with the default pidfile path unchanged.

The motivation for this feature comes from using Docker to run multiple
instances of the same rails app. When developing a rails app with
Docker, it's common to bind-mount the rails root directory in the
running container, so that changes to files are shared between the
container and the host. However, this doesn't work so well with the
pidfile and it's necessary to (remember to) add a '-P' flag to the
'rails server' command line; being able to specify this flag using an
env var would make developing with Rails+Docker a bit simpler.

Other Information

[y-yagi]

You don't need to add a section header.

[y-yagi]

Please use * and add your name. Please refer to other CHANGELOG files.
https://github.com/rails/rails/blob/master/activesupport/CHANGELOG.md
https://github.com/rails/rails/blob/master/activerecord/CHANGELOG.md

[y-yagi]

This change affected to an output of help. Please add a default value to desc like port and binding.

[y-yagi]

You should need to fix prepare_restart also.

rails/railties/lib/rails/commands/server/server_command.rb

Lines 263 to 265 in c8a8460

	def prepare_restart
	FileUtils.rm_f(options[:pid]) if options[:restart]
	end

[benthorner]

@y-yagi thanks for your comments - first time working on Rails and I missed a few things :-). I've addressed them all - couldn't find a particular test for the prepare_restart method, but I changed it to use the pid method now.

[y-yagi]

I missed to point out user_supplied_options. Can you fix to user_supplied_options whenPIDFILE env is specified?

rails/railties/lib/rails/commands/server/server_command.rb

Lines 208 to 210 in aeba121

	user_supplied_options << :Host if ENV["HOST"] || ENV["BINDING"]
	user_supplied_options << :Port if ENV["PORT"]
	user_supplied_options.uniq

[benthorner]

@y-yagi sure, done :-).

[y-yagi]

@benthorner Thanks!!