Azure CR plugin

[jpds]

It would be neat if this plugin had integration for Azure Container Registry.

There is some example code for the Azure SDK at: Azure-Samples/azure-sdk-for-go-samples#215

[jpds]

I managed to get this working with plain 'headless' docker and a service principal per the Microsoft documentation:

• https://docs.microsoft.com/en-us/azure/container-registry/container-registry-auth-service-principal

[bradrydzewski]

This is not something I plan to work on, but would be happy to accept a pull request.