What
---
- Update the `CMD` fo the gitlab-container-registry image to be in `exec
form` using the array notation rather then the shell form.
- Move the command into a shell script so variable expanstion works,
  because if we put in the `CMD` array it will not be expanded.

Why
---
In https://gitlab.com/gitlab-org/charts/gitlab/-/issues/3249 we are
going through all the images that have `CMD` defiend and updating it to
be from `CMD command` (shell form) to `CMD ["executable"]` (exec form)
so that we are consistent and the main process is always PID 1 so the
main process gets the termination signal.

Reference: https://gitlab.com/gitlab-org/charts/gitlab/-/issues/3249


Signed-off-by: Steve Azzopardi <sazzopardi@gitlab.com>