Thank you for the suggestions, but I had already bumped into these and they don’t work. /sysroot is not mounted to start with, and my fstab is missing (the only thing I have is a /etc/fstab.empty which is unsurprisingly empty ;-)). The os-release file is there and looks fine.
After investigating a bit more, the earliest problem that I can identify is this:
systemd-gpt-auto-generator: EFI loader partition unknown, exiting. (The boot loader did not set EFI variable LoaderDevicePartUUID)
Again, my google-fu is failing me and I have no idea how to proceed :-/