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.
Why not do this right in `Transport` initialization? like `TLSClientConfig: &tls.Config{InsecureSkipVerify: !secure}`?