specify user in rootless container numerically (#16361)

specify user in rootless container numerically (#16361)

* specify user in rootless container numerically

With kubernetes' PodSecurityPolicy set to runAsNonRoot it will not allow starting the container.  The error message is
```
Error: container has runAsNonRoot and image has non-numeric user (git), cannot verify user is non-root
```
The `USER` directive has to be numerical for that to work.

* mention the name of the uid/gid

Co-authored-by: 6543 <6543@obermui.de>