How to add unit tests to gitlab-ci?

Question

GitLab

Gitlab-CI is building a build built on Docker containers. But you need to add a run of unit tests. Those. enter the container, run unit tests and, depending on the results, either stop the build or continue. How to determine the ID of the desired container in order to get into it?

Answer 1

And what prevents you from creating a test image, after successful tests, reupload this image to the prod version ?!
For example, a piece from a pipe:

Step 3:
  stage: build
  only:
    - "master"
  script:
    - docker pull $IMAGE:${VERSION}_test
    - docker tag $IMAGE:${VERSION}_test $IMAGE:latest
    - docker tag $IMAGE:${VERSION}_test $IMAGE:$VERSION
    - docker push $IMAGE:latest
    - docker push $IMAGE:$VERSION
  tags: [build]