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.
Only because I didn't know for sure if initializing the `TLSClientConfig` field with a non-`nil` (but default-constructed) value could have side effects. But you're right, it shouldn't have any: updating.