How do I seed a mongo database using docker-compose?

I have found useful to use Docker Custom Images and using volumes, instead of creating another container for seeding.

File Structure

.
├── docker-compose.yml
├── mongo
│   ├── data
│   ├── Dockerfile
│   └── init-db.d
│       └── seed.js

Every File location mentioned in Dockerfile/docker-compose.yml, is relative to location of docker-compose.yml

DOCKERFILE

FROM mongo:3.6

COPY ./init-db.d/seed.js /docker-entrypoint-initdb.d

docker-compose.yml

version: '3'

services:
  db:
    build: ./mongo
    restart: always
    volumes:
      - ./mongo/data:/data/db #Helps to store MongoDB data in `./mongo/data`
    environment:
      MONGO_INITDB_ROOT_USERNAME: {{USERNAME}}
      MONGO_INITDB_ROOT_PASSWORD: {{PWD}}
      MONGO_INITDB_DATABASE: {{DBNAME}}

seed.js

// Since Seeding in Mongo is done in alphabetical order... It's is important to keep
// file names alphabetically ordered, if multiple files are to be run.

db.test.drop();
db.test.insertMany([
  {
    _id: 1,
    name: 'Tensor',
    age: 6
  },
  {
    _id: 2,
    name: 'Flow',
    age: 10
  }
])

docker-entrypoint-initdb.d can be used for creating different users and mongodb administration related stuffs, just create an alphabetical ordered named js-script to createUser etc...

For more details on how to customize MongoDB Docker service, read this

Also, it is good to keep your passwords and usernames secure from Public, DO NOT push credentials on public git, instead use Docker Secrets. Also read this Tutorial on Secrets

Do note, it is not necessary to go into docker-swarm mode to use secrets. Compose Files supports secrets as well. Check this

Secrets can also be used in MongoDB Docker Services

I do this using another docker container whose only purpose is to seed mongo, then exit. I suspect this is the same idea as ebaxt's, but when I was looking for an answer to this, I just wanted to see a quick-and-dirty, yet straightforward, example. So here is mine:

docker-compose.yml

mongodb:
  image: mongo
  ports:
    - "27017:27017"

mongo-seed:
  build: ./mongo-seed
  depends_on:
    - mongodb

# my webserver which uses mongo (not shown in example)
webserver:
  build: ./webserver
  ports:
    - "80:80"
  depends_on:
    - mongodb

mongo-seed/Dockerfile

FROM mongo

COPY init.json /init.json
CMD mongoimport --host mongodb --db reach-engine --collection MyDummyCollection --type json --file /init.json --jsonArray

mongo-seed/init.json

[
  {
    "name": "Joe Smith",
    "email": "[email protected]",
    "age": 40,
    "admin": false
  },
  {
    "name": "Jen Ford",
    "email": "[email protected]",
    "age": 45,
    "admin": true
  }
]