MongoDB authentication fails in Docker

My docker-compose.yml

  mongo:
    image: mongo
    volumes:
      - ./mongo:/data/db
    ports:
      - 27017:27017
    restart: always
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: pass

  mongo-express:
    image: mongo-express
    restart: always
    ports:
      - 8081:8081
    environment:
      ME_CONFIG_MONGODB_ADMINUSERNAME: root
      ME_CONFIG_MONGODB_ADMINPASSWORD: pass
      ME_CONFIG_MONGODB_URL: mongodb://root:pass@mongo:27017/

When I run docker-compose up it logs the following errors

for the mongo service: UnhandledPromiseRejectionWarning: MongoNetworkError: failed to connect to server [mongo:27017] on first connect [MongoError: Authentication failed.

and for the mongo-express service: "error":"UserNotFound: Could not find user "root" for db "admin"

I tried adding ?authSource=admin to the ME_CONFIG_MONGODB_URL but the error remains.