It probably depends on your distro of interest, but lots of tools exist from even VM image creation days to create root filesystems. For example, if you find a "cloud image" build of various distros as a tarball, this can probably suffice as a starting point. For specific tools, see "debootstrap" for Debian; search on rkt or appc rootfs bundle creation or look up the "fakeroot" concept. In general, you should find various methods depending again on your base image layer interest (e.g. Fedora, Debian, Ubuntu, Alpine, Busybox, etc.)