Send your test sources to /tests with a volume, or a COPY.

You can then run any codecept command using the image. Here is an example of complex command using docker-compose:

docker-compose run --rm codeception codecept run -c offline  --steps --no-ansi

If you want to run acceptance tests on a real url, you can link containers to codeception in order to access them from your tests.

Here is an example using docker-compose:

codeception:
  image: occitech/codeception
  links:
    - web:site.test
    - web:en.site.test
  volumes:
    - .:/tests