turns out that scratch is empty, very empty.
RUN useradd would execute /bin/sh -c useradd but there is no /bin/sh . RUN ["useradd"] would exec directly. but there is no useradd. i d have to add rootfs.tar and build stuff from zero.
i ll use debian as i don't wont to run anything as root within a container because ...
Treat root within a container as if it is root outside of the container