kubeval dosn't check for lowercase image names

I'm getting the following error:
```
CrashLoopBackOff: back-off 5m0s restarting failed container=nginx pod=foo-5b9957fff9-f6hqp_bar-staging(92483072-df8f-4fce-bbd7-3a6f2096691a)
InvalidImageName: Failed to apply default image tag "${PHP_FPM_IMAGE}": couldn't parse image reference "${PHP_FPM_IMAGE}": invalid reference format: repository name must be lowercase
```

While the actual error was not correctly replacing the variable in our template, the error could well have been caught bu static checking.

I'm not sure though if this is a kubernetes or kubeval issue.