$ fluentd -c in_docker.conf
2015-09-01 15:07:12 -0600 [info]: reading config file path="in_docker.conf"
2015-09-01 15:07:12 -0600 [info]: starting fluentd-0.12.15
2015-09-01 15:07:12 -0600 [info]: gem 'fluent-plugin-mongo' version '0.7.10'
2015-09-01 15:07:12 -0600 [info]: gem 'fluentd' version '0.12.15'
2015-09-01 15:07:12 -0600 [info]: adding match pattern="*.*" type="stdout"
2015-09-01 15:07:12 -0600 [info]: adding source type="forward"
2015-09-01 15:07:12 -0600 [info]: using configuration file: <ROOT>
2015-09-01 15:07:12 -0600 [info]: listening fluent socket on 0.0.0.0:24224