`docker:dind` and expose ports

Hi!

I have a problem with running the CI process. I would like to run docker-compose stack. Something like this:

Behat:
  stage: Test
  image:tmaier/docker-compose:latest
  before_script:
    - docker-compose -f docker-compose.yml up -d
  script:
    - docker-compose -f docker-compose.yml exec -T php vendor/bin/behat 

I use docker runners. But there is a problem: MySQL port is already allocated. This is true, but on a host machine and I thought that I run this stack in isolation?