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.
I'm wondering if this parameter should be something like "verify", inferring that it only disables ssl verification. `secure` as a boolean opens up the ability for it to gain other functionality.