Add STOPSIGNAL to gitlab-runner docker images

What does this MR do?

Instructs docker to stop the gitlab-runner docker images using SIGQUIT instead of the default SIGTERM.

Why was this MR needed?

On a busy system with many runners using the gitlab-runner image, it can be difficult to upgrade the image without killing running builds.

With this change you can trigger an upgrade and know current builds will finish before the container quits and the upgrade happens.

For cases where the container must die within a timeout, parameters such as "terminationGracePeriodSeconds" in Kubernetes can be used.