Connecting Spring boot Container to Mongo Container: java.net.UnknownHostException: mongo: System error?

I have created a simple Spring Boot Rest Application that I have dockerized and is working as expected when I use mongo locally.

However I have now tried to use Docker Compose to run a Mongo container to connect to rather than my local host.

Docker-compose up -d works but when I try to hit my endpoint that queries Monogo I get:

Caused by: java.net.UnknownHostException: mongo: System error
    at java.base/java.net.Inet4AddressImpl.lookupAllHostAddr(Native Method) ~[na:na]
    at java.base/java.net.InetAddress$PlatformNameService.lookupAllHostAddr(Unknown Source) ~[na:na]
    at java.base/java.net.InetAddress.getAddressesFromNameService(Unknown Source) ~[na:na]
    at java.base/java.net.InetAddress$NameServiceAddresses.get(Unknown Source) ~[na:na]
    at java.base/java.net.InetAddress.getAllByName0(Unknown Source) ~[na:na]
    at java.base/java.net.InetAddress.getAllByName(Unknown Source) ~[na:na]
    at java.base/java.net.InetAddress.getAllByName(Unknown Source) ~[na:na]
    at com.mongodb.ServerAddress.getSocketAddresses(ServerAddress.java:203) ~[mongodb-driver-core-3.11.2.jar!/:na]

In my application.properties file I have:

spring.data.mongodb.uri=mongodb://mongo:27017/pokerStats

My Docker file for my spring boot app (called pokerStats):

#
FROM adoptopenjdk/openjdk11:alpine-jre

#
ARG JAR_FILE=/build/libs/pokerStats-0.0.1-SNAPSHOT.jar

#
WORKDIR /opt/app

#
COPY ${JAR_FILE} app.jar

#
ENTRYPOINT ["java","-jar","app.jar"]

My docker-compose.yml file:

version: "3"
services:
  pokerStats:
    image: pokerStats
    ports:
      - 8080:8080
    depends_on: 
      - db
  db:
    image: mongo
    volumes:
      - ./database:/data
    ports:
      - "27017:27017"

The properties file specifies the host mongo, but containers are discoverable via the container name, which is db.

Either the properties need to be changed to use the right name or the container name changed to what the properties file expects.