Is the docker image really like new in the next gitlab-ci build run?


I am using gitlab-ci on some of our repositories on Gitlab CE 8.10.2
The executor type is docker and the builds are executed without any problems.

Within our C++ project we automatically install correctly defined dependencies via apt-get.
My question now is this: When I run the build the next time, are those packages already installed or not?

To be sure that our dependencies are correctly defined I hope that the docker container does not have them installed and the build would fail.
I thought this should be like this (clear and empty image), however I observe in the build logs that certain directories which are within our repo are being deleted before it is cloned. As this is not defined within my gitlab-ci.yml file I am not sure why it would try to delete those files, or how it would even know that those files could exist.

Maybe someone can shed a little light on to this issue.
Thanks a lot.