The docker-dind Dockerfile expose both port 2375 and port 2376:
We had dockerd configured to only listen to 2375, while GitLab CI/CD
waits for all ports. When dockerd didn't listen on port 2376 GitLab
waits for 30 seconds, then logs a warning.
This commit adds the second port which silences the warning message
and makes the job 30s faster.