CircleCI - Running built docker image tests with additional database image - Connection refused

Question

I wrote a circleci config for creating docker image of my app, running it, running tests, and pushing it to DockerHub.

But I can't figure out how to run tests that require a database.

Here is part of my config for running tests.

executors:
    docker-executor:
        environment:
            DOCKER_BUILDKIT: "1"
        docker:
            - image: cimg/base:2021.12

jobs:
    run-tests:
        executor: docker-executor
        steps:
            - setup_remote_docker:
                version: 20.10.7
                docker_layer_caching: true
            - run:
                name: Load archived test image
                command: docker load -i /tmp/workspace/testimage.tar
            - run:
                  name: Start Container
                  command: |
                      docker create --name app_container << pipeline.parameters.app_image >>:testing
                      docker start app_container
            - run:
                  name: Run Tests
                  command: |
                      docker exec -it app_container ./vendor/bin/phpunit --log-junit testresults.xml --colors=never

How to add MySQL service here, and how to connect it with my app docker image so I can run tests that require database?