code: function(command, arg) {
      if (!arg) {
        arg = 'default';
      }
  
      if (!commands[command]) {
        grunt.fail.fatal('Command [' + command + '] not found.');
      }
  
      // Check arg
      if (typeof (commands[command]) !== 'function') {
        if (!commands[command][arg]) {
          grunt.fail.fatal('Argument [' + arg + '] for [' + command
              + '] not found.');
        }
      }
  
      var func = (arg) ? commands[command][arg] : commands[command];
      if (!func) {
        func = commands[command]; // fallback to the main function
      }
  
      var options = this.options();
      var docker = (options.docker) ? new Docker(options.docker) : null;
      var done = this.async();
  
      var callback = function(e) {
        if (e) {
          grunt.fail.warn(e);
        }
        done();
      };
  
      func.apply(this, [ grunt, docker, options, callback, arg ]);
    }
docs: Process the given command with arg.