Configuring the Docker runner to auto-add to Gitlab when CI_SERVER_URL and REGISTRATION_TOKEN are present.

Being able to dynamically add runners without configuration is highly desirable for a CI environment. With a minor tweak to the Dockerfile, the runner is now able to infer its environment and launch with very little intervention.

I expect that there will be documentation additions necessary for this change, but I wanted to make sure that the change was applicable to Gitlab prior to adding documentation. I'm very willing to add documentation and whatever else is necessary if it means integrating this change.

Please let me know if you have questions or concerns. Thanks!