I think the script is named start_desktop.sh not just start_desktop. But other than that, there seems to be an issue with Sid now, as xf86vidmodeproto is a required package but is now missing from the unstable repository, which was not the case when we set up our Sid containers here. It should work if you can find a way to install it on Sid (unless there are other issues downstream), or alternatively you could try installing Buster instead, but you'll lose the benefit of having a rolling distributions. You can try Debian Testing too, I'm not sure if it's rolling or discontinuous releases.