Allow admins to override image:kubernetes:user value
## Description
Recently gitlab-org/gitlab-runner!5469 was merged that allows users to define a uid and gid for a container being run in kubernetes. From the MR, the user setting overrides the value configured by administrators. Instead, administrators should be able to supersede any user-defined options.
## Proposal
Reverse the order of precedence for the `image:kubernetes:user` and `image:kubernetes:group` values so that administrators can lock the jobs being run to a single user namespace.
From customer feedback:
> Ideally we want a setting like:
```
allowed_users = [">1", "1000-2000"]
allowed_groups ["0", ">1000"]
```
## Links to related issues and merge requests / references
gitlab-org/gitlab-runner!5469
issue