Pull helper image from GitLab registry by default

What does this MR do?

Pull helper image from GitLab registry by default

Why was this MR needed?

In !2554 (merged) a feature flag was introduced to allow users to pull the helper image from GitLab Registry where no DockerHub limit apply. This was off by default, this commit enables it by default since it will land in the 14.0 release.

What's the best way to test this MR?

Linux Docker Executor

1. Start a job with the following .gitlab-ci.yml

   .gitlab-ci.yml

   job:
     stage: test
    script:
    - sleep 300

2. docker rmi registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-latest

3. Make sure you don't have the helper image in a known directory for development rm -rf out/helper-images/

4. Start the runner for example with go run main.go run -c config.toml

   config.toml

   [[runners]]
     name = "docker"
     url = "https://gitlab.com/"
     token = ""
     executor = "docker"
     [runners.docker]
       image = "alpine:3.13"

5. Run the job

6. You see the following inside of the job trace

7. docker rmi gitlab/gitlab-runner-helper:x86_64-latest

8. Disable FF_GITLAB_REGISTRY_HELPER_IMAGE

9. You should see the following inside of the job trace

What are the relevant issue numbers?

Closes #27218 (closed)