Clarify need for entrypoint to open a shell
In gitlab-org/gitlab-runner!3095 (merged), we are fixing a bug so that a docker image's entrypoint is not ignored when running CI jobs.
However, we need to clarify that a custom entrypoint must open a shell or else the CI job will hang forever.
ee/ci/docker/using_docker_images.md already has some details about this, but we need to spell it out more clearly.