authSource=admin
が必要。
# Ruby
client = Mongo::Client.new('mongodb://root:example@mongo:27017/mydb?authSource=admin')
なお、docker-compose.ymlはこんな感じで書いてある想定。
# Use root/example as user/password credentials
version: '3.1'
services:
mongo:
image: mongo
restart: always
environment:
MONGO_INITDB_ROOT_USERNAME: root
MONGO_INITDB_ROOT_PASSWORD: example