The docker client allows for specifying a configuration directory that contains the docker client's configuration. It is common to store "docker login" credentials in this config, to avoid the need to docker login on each cluster member.
By default the docker client config is $HOME/.docker/config.json on Linux. However, this does not work with the current container executor user switching and it may also be desirable to centralize this configuration beyond the single user's home directory.
Note that the command line arg is for the configuration directory NOT the configuration file.
This change will be needed to allow YARN to automatically pull images at localization time or within container executor.