helper_image - Allow overriding registry URL

I'd like to override only the registry.gitlab.org portion of the helper_image. I know you can set the full helper_image, but then you cannot dynamically source the tag. The closest I've found thus far is passing ${CI_RUNNER_VERSION} but this still leaves me with a static architecture.

It would be nice if helper_image_autoset_arch_and_os did this if it detected a missing tag in helper_image, or perhaps there could be a seperate field helper_registry added.

[runners.kubernetes]
  helper_image = "artifactory.my.org.net:443/gitlab-registry-proxy/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-v${CI_RUNNER_VERSION}"