Allow login to insecure registry

Enable TLS `InsecureSkipVerify` for `docker login` when the targeted registry is specified in `--insecure-registry`.

Fixes #9752.
Reading through the code, it might also fix an unreported issue when using `login` behind a proxy.

Ping @dkjer @tiborvass.
You're right it's not the best name. I chose it because I found it with the exact same meaning [here](https://github.com/docker/docker/blob/d09421a74fa268274c50308f2655e654949f505b/registry/registry.go#L38) (which is basically part of the code path used for all other calls to the registry as far as I understand).