add back validation (maybe?) for `disable registry url validation in prune`

[deads2k]

This was removed in disable registry url validation in prune

reenable

if _, err := url.Parse(o.RegistryUrlOverride); 
   err != nil { in pkg/cmd/admin/prune/images.go

[mfojtik]

@deads2k @miminar ideally I would drop that option in favor of https://trello.com/c/2cHkrqIu/954-provide-public-pull-url-for-images

[deads2k]

@deads2k @miminar ideally I would drop that option in favor of https://trello.com/c/2cHkrqIu/954-provide-public-pull-url-for-images

Your call. Presumably the code you're calling will do something reasonable.

[miminar]

We need to support the flag anyway. Even the best algorithm we can make cannot be right all the time.

Therefor the validation should be re-enabled. The argument isn't really a URL, it's rather a shortened pull-spec (just the registry part). I thought we could use docker's validation, but they still use url.Parse(). And their validation for insecure registries is private. I'll come up with something.

[miminar]

Addressed by #14914