Meu Application.properties está assim:
spring.application.name=crud-mongo
spring.data.mongodb.host=mongo
spring.data.mongodb.port=28000
spring.data.mongodb.database=crud
spring.data.mongodb.username=root
spring.data.mongodb.password=root
spring.data.mongodb.authentication-database=admin
Meu docker-compose assim:
version: '3.8'
services:
  mongo:
    image: mongo:latest
    container_name: mongo
    restart: always
    ports:
      - 28000:27017
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: root
    volumes:
      - mongo-data:/data/db
volumes:
  mongo-data:
O container está rodando normalmente:
  
E está dando esse erro:
com.mongodb.MongoSocketException: Este host não é conhecido (mongo)
    at com.mongodb.internal.connection.ServerAddressHelper.getSocketAddresses(ServerAddressHelper.java:75) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.SocketStream.initializeSocket(SocketStream.java:100) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.SocketStream.open(SocketStream.java:79) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.InternalStreamConnection.open(InternalStreamConnection.java:233) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.DefaultServerMonitor$ServerMonitor.lookupServerDescription(DefaultServerMonitor.java:219) ~[mongodb-driver-core-5.2.1.jar:na]
    at com.mongodb.internal.connection.DefaultServerMonitor$ServerMonitor.run(DefaultServerMonitor.java:176) ~[mongodb-driver-core-5.2.1.jar:na]
Caused by: java.net.UnknownHostException: Este host não é conhecido (mongo)