support for environment variable for puma PORT

config/puma.rb

@@ -2,7 +2,7 @@ require_relative '../lib/postal/config'
 threads_count = Postal.config.web_server&.max_threads&.to_i || 5
 threads         threads_count, threads_count
 bind_address  = Postal.config.web_server&.bind_address || '127.0.0.1'
-bind_port     = Postal.config.web_server&.port&.to_i || 5000
+bind_port     = ENV['PORT'] || Postal.config.web_server&.port&.to_i || 5000
 bind            "tcp://#{bind_address}:#{bind_port}"
 environment     Postal.config.rails&.environment || 'development'
 prune_bundler