Requirements to boot a rootfs.img file

I have a Linux image from which I've extracted a rootfs.img file. The file seems to contain files and information. I would like to "boot into it". File contents:

bin
dev
etc
home
lib
media
mnt
proc
run
sbin
service
sys
tmp
usr
var
lib64

How can I install a bootloader to boot it? I've tried getting a live Ubuntu image, adding a partition, and placing the contents of rootfs.img inside the partition and running "Boot Repair", which did find the other system as another bootable Linux, but I can't boot into it, it seems like I'm missing something