Docker container to container connect: connection refused

When all are run standalone outside of docker it works with no problem when core attempts to do a get from cerner. However, doing the same when all are dockerized as below I get:

Get http://cerner:8602/api/v1/patient/search: dial TCP 192.168.240.4:8602: connect: connection refused. The .4 is the IP of the cerner container and .2 is the IP of the core container

Cerner is the name of the container being called from core. If I change the name to the ip-address of the host server and use the ports, it works fine also. It just does not allow container to container using the containers DNS or IP. I have attempted with and without the private network and get the same thing.

The containers are all scratch go.

version: '3.7'

services:    caConnector:
      image: vertisoft/ca_connector:latest
      ports:
        - "8601:7001"
      env_file:
        - .env.ca_connector
      networks:
        - core-net

   fhir:
      image: vertisoft/fhir_connector:latest
      container_name: cerner
      ports:
        - "8602:7002"
      env_file:
        - .env.fhir_connector
      networks:
        - core-net

   core:
      image: vertisoft/core:latest
      ports:
        - "8600:7000"
      env_file:
        - .env.core
      networks:
        - core-net

networks:   core-net:
    driver: bridge

Source: StackOverflow