Thank you for your feedback and suggestion.
	I tried Go's suggestion but it does not work. I've added exactly the line into config/additional_environment.rb but the IP address does not show up in the log.
	My setup is a docker compose stack using the docker image redmine:4-passenger currently running redmine version 4.1.1.stable where I'm using the log file /usr/src/redmine/log/passenger.3000.log.
	I've added the line suggested by Go into config/additional_environment.rb and mapped that file to /usr/src/redmin/config/additional_environment.rb added permissions and file ownership par to config/environment.rb, checked if the setting are there from whithin the running container. Now the content of config/additional_environment.rb is as follows:
config.logger = Logger.new(STDOUT)
config.log_tags = config.log_tags.to_a + [:remote_ip]
	I've managed to add some code in config/environment.rb changing the timestamp in the log lines via
class Logger
  def format_message(severity, timestamp, progname, msg)
    "#{timestamp} (#{$$}) #{msg}\n" 
  end
end
which works as expected.
	
Currently, with the addition from Go in config/additional_environment.rb, the log output in passenger.3000.log looks like this:
App 199 output: 2021-08-24 09:31:06 +0200 (199) Started GET "/users/current.xml" for 136.243.54.73 at 2021-08-24 09:31:06 +0200
App 199 output: 2021-08-24 09:31:06 +0200 (199) Processing by UsersController#show as XML
App 199 output: 2021-08-24 09:31:06 +0200 (199)   Parameters: {"id"=>"current"}
App 199 output: 2021-08-24 09:31:06 +0200 (199) User find_by_api_key 'dfasdfasdfasdf' gefunden: ''
App 199 output: 2021-08-24 09:31:06 +0200 (199)   Current user: anonymous
App 199 output: 2021-08-24 09:31:06 +0200 (199) Filter chain halted as #<Proc:0x0000564c7bb5e460@/usr/src/redmine/app/controllers/users_controller.rb:25 (lambda)> rendered or redirected
App 199 output: 2021-08-24 09:31:06 +0200 (199) Completed 401 Unauthorized in 9ms (ActiveRecord: 3.7ms)