The link has already been provided.
The runner is only an administrative process. It can be indeed put into a Docker container. The instructions are easy to follow, the image is built by gitlab and updated regularly. If you choose the alpine variant instead of latest it's much smaller. We have done that for 2+ years and not noted any limitations.
The runner will start jobs that do the real building (or whatever you have coded in your CI). Again you can choose to have jobs executed in docker containers. Also documented by gitlab, easy to set up. Of course you need to provide a suitable Docker image where your build can work. Nobody can do that for you. In simple cases you can pull something existing from Docker hub without further additions.
The runner will start jobs that do the real building (or whatever you have coded in your CI). Again you can choose to have jobs executed in docker containers. Also documented by gitlab, easy to set up. Of course you need to provide a suitable Docker image where your build can work. Nobody can do that for you. In simple cases you can pull something existing from Docker hub without further additions.