How to utilize a Mongo seed container to import users to admin?

Question

Mongo seed container comes in handy when we need to pre-insert some collections before initializing a mongo container. For instance, like the solution in this question:How do I seed a mongo database using docker-compose?

But what about importing users to admin database with the following Docker set up:

1. docker-compose.yml:

   version: '3.4'
   services:
     mongodb:
       image: mongo:latest
       ports: 
       - "27017:27017"
       environment:
       # provide your credentials here
       - MONGO_INITDB_ROOT_USERNAME=root
       - MONGO_INITDB_ROOT_PASSWORD=root
       container_name: mongodb 
     mongodb_seed:
       build: mongodb_seed
       links:
       - mongodb
   

   with this setting, mongodb_seed can be used to insert ready data into mongodb right before initializing

2. Dockerfile of mongodb_seed:

   FROM mongo:latest
   WORKDIR /tmp
   COPY users.json .
   COPY insert_users.js .
   COPY import.sh .
   CMD ["/bin/bash", "-c", "source import.sh"]
   

   with this setting, the import.sh can be used to execute the insert_users.jscontaining code like:

   use admin
   db.createUser()
   

   in this case the import.sh might look like:

   #!/bin/bash
   mongo -u root -p root --authenticationDatabase admin insert_users.js
   

But this approach got connection refused in the log:

    mongodb_seed_1  | connecting to: mongodb://127.0.0.1:27017/? authSource=admin&gssapiServiceName=mongodb
    mongodb_seed_1  | 2019-04-03T07:39:27.902+0000 E QUERY    [js] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed: SocketException: Error connecting to 127.0.0.1:27017 :: caused by :: Connection refused :
    mongodb_seed_1  | connect@src/mongo/shell/mongo.js:343:13
    mongodb_seed_1  | @(connect):2:6
    mongodb_seed_1  | exception: connect failed

How to resolve this issue?