kubeval dosn't check for lowercase image names

[frederik-b]

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.