I installed the SlowCheetah package via nuget and added transform files for my web.config based on Build config. However, on building, the web.config does not get transform
To elaborate on Philipp's answer I have found a possible simpler solution: No need for a Web.Base.Config and no problems with overwriting your web.config which causes problems in Source Control.
BeforeBuild: You make the destination your TargetDir en TargetFileName.
AfterBuild: You copy this to your published websites after the build is done.