Warn when parsing local config that refers to unsupported external credentials stores

[declension]

Description

This is very related to #657 (and originally dockerfile-maven #22).

The idea here is that a significant improvement for the end user is to simply recognise that their config.json uses an external credentials store that won't be supported, so fail fast, or at least output a warning message (e.g. it took me a long time to work out why my "image wasn't found" when the plugin despite docker build working fine when building the same image).

How to reproduce

Use a recent Docker client, that defaults to using the new secure external credentials store (OSX keychain, Windows Credentials store, D-Bus store etc)

What do you expect

Either for an execption to be thrown, or some sort of warning message about credentials not found / understood.

What happened instead

The client continues to authenticate with a null token, and fails (of course)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.