Currently, I am able to the same but with only one container by assigning static ip addresses to the container. Not sure how to implement this for multiple ones. Each rtu needs to have a diff IP adress and be associated with a ctrl unit. This ctrl unit is then connected to a network, linked to a gateway.

services:
  rtu_a:
    logging:
      driver: "json-file"
      options:
        max-size: "200k"
        max-file: "10"
    image: ${REGISTRY_PREFIX}testbed/modbus${REGISTRY_TAG}
    volumes:
      - ./volume:/var/lib/my_volume
    build:
      context: .
      cache_from:
        - ${REGISTRY_PREFIX}modbus${REGISTRY_TAG}
        - ${REGISTRY_PREFIX}modbus:${CI_COMMIT_REF_SLUG}
        - ${REGISTRY_PREFIX}modbus:master
    networks:
      rtu-ctrl-net-a:
        ipv4_address: 172.31.0.101
    command: python3 rtu_unit.py "rtu_a"

  ctrl_unit_a:
    logging:
      driver: "json-file"
      options:
        max-size: "200k"
        max-file: "10"
    image: ${REGISTRY_PREFIX}testbed/modbus${REGISTRY_TAG}
    volumes:
      - ./volume:/var/lib/my_volume
    depends_on:
      - rtu_a
    networks:
      rtu-ctrl-net-a:
        ipv4_address: 172.31.0.102
    command: python3 ctrl_unit.py rtu_a 1 "ctrl_unit_a"