Thanks a lot for posting this great suggestion!
Windows environments is something we’ve wanted for a long time (in fact, it was the 3rd issue opened on our GitHub repository), but since it’s not trivial how to make it work on Janitor, we haven’t made significant progress on this issue until recently.
You make a good point about needing a GUI subsystem, but I don’t think the microsoft/windowsservercore image actually has one. The abscence of any graphical environment lead us to discard Windows Containers as an option for now, and instead we’re currently investigating Windows VMs on Azure and AWS (Mozilla already uses AWS for its Firefox Windows continuous integration, so maybe we can re-use some of their work on this).
Here is a shared document with some notes on our progress (TLDR in parallel, we’re trying to build a Firefox VM image on Azure, and also looking into using TaskCluster workers to get Windows environments on AWS): https://docs.google.com/document/d/13EPZRRSAfHwhs42Q9M392HjfZ9wzgs4H9wUR7uxD_So/edit?usp=sharing
Note: Windows support for Janitor recently became a top priority at Mozilla, because this could be very helpful for Firefox developers (because they generally already have a working Linux or Mac environment, but struggle with VMs or dual boots when they need to fix Windows bugs).