Building a project with Gitlab CI, Docker, Qt 5.6+ and a very strange structure, can anyone help or suggest?

Question

Qt

In the organization where I work as a system administrator, there was a need to deploy a project for a "development system".
I am required to:

1. The user should be able to get the project folder from the server and upload the changed data to it.
   
2. Next, a CI process should be launched to build and test the product in order to improve quality and fix bugs.
   

I organized the first possibility by bringing up a gitlab, which includes git, and giving access to it.
The second part raised a number of questions:

1. The project structure consists of n subprojects that run independently of each other. In my opinion, it will not work to test such a system using CI, except for the case when each subproject will have functionality similar to qDebug for displaying information.
   
2. I don’t quite understand how exactly the project is built in Docker or Gitlab CI, as I understand it, the environment that is necessary for the operation of all subprojects must also be deployed in the container or on the Gitlab server. That is, at least Qt5.6+, mingw, git. In order to run runner, you need to install it on docker and connect it to the gitlab server.
   At the same time, this approach to project development surprises me a little, since I don’t understand how CI will help improve the quality of the product and testing of this product.
   If anyone can help I would be very grateful.