Add servercore:ltsc2019 image

[hbuckle]

With the removal of the 1809 image there doesn't appear to be an image that will work with Azure Pipelines anymore, as the hosted agents are running on Server 2019 (10.0.17763).
Can an image based on servercore:ltsc2019 be added?

[failshell]

Same problem on Github Actions runners, which are Azure Pipelines agents.

[MatteoJoliveau]

Same problem when running on an AWS EC2 instance with AMI Windows_Server-2019-English-Full-ContainersLatest, which runs version 10.0.1763 as well.

[TravisEz13]

ltsc2022 was added. We can probably go back and add a ltsc2019 if it is still needed. cc @anamnavi

[rgl]

@TravisEz13 where was ltsc2022 built? GitHub Actions does not yet seem to have a windows 2022 virtual environment.

[TravisEz13]

@rgl a private azure environment not currently available to the public.

[jpatigny]

+1 Gitlab runners doesn't officially support windows 2022 yet...

[rgl]

@jpatigny, Windows 2022 is already in beta, so its somewhat supported.

[TravisEz13]

We build in ACR, we only use the agent to coordinate the build.

[amitie10g]

I'm building an image that depends on Nano Server. Altrough this is related to Server Core, no images for ltsc2019 (both Server Core and Nano Server) are available, even if Nano Server ltsc2019 is still available and mainstream supports ends at 2024 and extended support ends at 2029.

I see no reasons to missing ltsc2019, existing ltsc2016, so, I thing ltsc2019 (based on both Server Core and Nano Server) should be available asap.