code: def _compose_dict_for_nginx(port_specs):
      
      spec = {'image': constants.NGINX_IMAGE,
              'volumes': ['{}:{}'.format(constants.NGINX_CONFIG_DIR_IN_VM, constants.NGINX_CONFIG_DIR_IN_CONTAINER)],
              'command': 'nginx -g "daemon off;" -c /etc/nginx/conf.d/nginx.primary',
              'container_name': 'dusty_{}_1'.format(constants.DUSTY_NGINX_NAME)}
      all_host_ports = set([nginx_spec['host_port'] for nginx_spec in port_specs['nginx']])
      if all_host_ports:
          spec['ports'] = []
          for port in all_host_ports:
              spec['ports'].append('{0}:{0}'.format(port))
      return {constants.DUSTY_NGINX_NAME: spec}
docs: Return a dictionary containing the Compose spec required to run
      Dusty's nginx container used for host forwarding.