In-repo sounds fine for a quickbuild.bat type of script, but remember that this will add another layer of indirection and potential confusion (“I tried the quickbuild.bat but it kept rebuilding everything from scratch and failing after 3 hours! Then I tried the superbuild instructions directly”, etc.).
I think providing a link (or automatic download) of correct Qt binaries would go further to simplify the development process on Windows. The current 5.9.2 official binaries ship both release and debug DLLs + PDBs, however Creator and Designer appear to only be built in release .
Probably best to keep Docker scripts outside of the repository as they are now.
I agree that it would be better not to add one more automation layer on top of CMake for building Slicer on a developer computer (fully automated Docker builds is a different story). To make the build process more robust, I would prefer to improve CMake scripts and/or documentation.