Add default `ENTRYPOINT` to Kubernetes Runner Helper images

Description

The GitLab Runner Helper image for Kubernetes Runners doesn't seem to have an ENTRYPOINT baked into it, as the Docker one does: docker.go#L181 <> kubernetes.go#L815.

docker inspect gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-v15.1.1 | grep -i entrypoint 
"Entrypoint": null, 
"Entrypoint": null,

Proposal

If my understanding is correct, the need for this use case was identified and fulfilled with: Add an ENTRYPOINT script to the helper image Dockerfiles to add CA certificates.

The same needs to be done for the Kubernetes Helper images.

Links to related issues and merge requests / references

• #3371 (closed)
• !2058 (merged)