text: Title: Can't connect to mongo docker
  Body: okey, spend a day, but still can't make this work
  
  > .env
  
  ```
  # Mongo DB
  MONGO_INITDB_ROOT_USERNAME=admin-user
  MONGO_INITDB_ROOT_PASSWORD=admin-password
  MONGO_INITDB_DATABASE=container
  ```
  
  > docker-compose.yml
  
  ```
   mongo-db:
      image: mongo:4.2.3
      env_file:
        - .env
      ports:
        - 27017:27107
      volumes:
        - ./bin/mongo-init.js:/docker-entrypoint-initdb.d/mongo-init.js:ro
      #    restart: always add this production
  
  
    api:
      build:
        context: ./backend
        dockerfile: Dockerfile
      command: uvicorn app.main:app --host 0.0.0.0 --port 8006 --reload
      volumes:
        - ./backend:/app
      env_file:
        - .env
      depends_on:
        - mongo-db
      ports:
        - "8006:8006"
  ```
  
  > mongo-init.js
  
  ```
  db.auth('admin-user', 'admin-password')
  
  db = db.getSiblingDB('container')
  
  db.createUser({
      user: 'test-user',
      pwd: 'test-password',
      roles: [
          {
              role: 'root',
              db: 'admin',
          },
      ],
  });
  
  ```
  using this as example - https://frankie567.github.io/fastapi-users/configuration/full_example/
  
  changed few lines:
  `DATABASE_URL = "mongodb://test-user:test-password@mongo-db/container"`
  ```
  client = motor.motor_asyncio.AsyncIOMotorClient(DATABASE_URL)
  db = client["container"]
  collection = db["users"]
  user_db = MongoDBUserDatabase(UserDB, collection)
  ```
  so think Fatapi is linked to Mongo container
  ![image](https://user-images.githubusercontent.com/17315578/80109217-e78acf80-8585-11ea-9008-2dcfb4de3667.png)
  
  
  but when i try to register user using Fastapi docks section, i get 
  "Internal server error"
  
  what I left unfinished?
  