code: def add_extra_container(self, container, error_on_exists=False):
          
          if container in self.extra_containers:
              if error_on_exists:
                  raise DatacatsError('{} is already added as an extra container.'.format(container))
              else:
                  return
  
          self.extra_containers.append(container)
  
          cp = SafeConfigParser()
          cp.read(self.target + '/.datacats-environment')
  
          cp.set('datacats', 'extra_containers', ' '.join(self.extra_containers))
  
          with open(self.target + '/.datacats-environment', 'w') as f:
              cp.write(f)
docs: Add a container as a 'extra'. These are running containers which are not necessary for
          running default CKAN but are useful for certain extensions
          :param container: The container name to add
          :param error_on_exists: Raise a DatacatsError if the extra container already exists.