Add support for Windows Server 1809

[kvonbredow]

Adding support for the new 1809 images, and following the base images to mcr.microsoft.com as described in the blog post here: https://azure.microsoft.com/en-us/blog/microsoft-syndicates-container-catalog/

[tianon]

This registry change is really a strange/confusing move by Microsoft IMO. 😞 (They were already hosting the layers, why add additional user-facing redirection? What's the benefit to users?)

I tried setting up a new Jenkins build worker to support this on Server 2019, but it appears there have been some changes that I haven't had a chance to dig into yet such that with a default install of OpenJDK just like we've done on previous Semi-Annual and LTSC releases causes errors with the Jenkins agent starting.

There's also the issue of LTSC 2019 and 1809 being the same thing in the Docker images (since they're core-only and that's the primary differentiator), so we need to decide whether we're going to call it 1809 or whether we'll use ltsc2019 instead. I am leaning towards 1809 since it matches our majority better.

[tianon]

Ok, given docker-library/official-images#5239 and the 2019 build server I managed to get working, I think this is good to go. 🤘

[tianon]

See also docker-library/hello-world#54, which might be worth putting through first/concurrently.