a .gitlab-ci.yaml for runner setup

Imagine you have a (shared) runner already and would be able therefore to install this project runner with the execution of a pipeline based on an .gitlab-ci.yaml which includes all cmds running based on public docker images to install all what is required and execute the cdk-fargate-gitlab-runner setup.

Start with the proposed .gitlab-ci.yaml and inspect the repository's cdk-fargate-gitlab-runner setup entry point and required commands. Define the public Docker images and pipeline steps needed to install and execute the setup, then verify that the pipeline completes successfully on a shared GitLab runner.

Written by the indexing model from the issue text.