build/

The build directory contains all the files you need (except the linux kernel source) to create your own local setup for testing.

Cloning the repo and applying the patch

cd build/
git clone https://github.com/torvalds/linux/
cd linux
git checkout eb7081409f94a9a8608593d0fb63a1aa3d6f95d8
git apply ../sys_jail.patch

Compiling the kernel

make menuconfig  # You don't need to configure anything. Just leave the default there and exit. This should create a .config file.
make bzImage -j $(nproc)
# You might need to set 
# CONFIG_SYSTEM_TRUSTED_KEYS="", CONFIG_SYSTEM_REVOCATION_KEYS="",  
# CONFIG_DEBUG_INFO_BTF=n
# in your .config if there is no bzImage in arch/x86/boot/ after the make and rerun the command above.
cp arch/x86/boot/bzImage ../

Launching

You can modify the used filesystem by modifying files in the fs/ directory and running ./rebuild_fs.sh afterwards.
./launch.sh will invoke qemu with the built kernel and the filesystem.
./go.sh is just a shortcut for ./rebuild_fs.sh and ./launch.sh.

deploy/

The deploy directory contains the complete remote setup (flag.txt in initramfs.cpio.gz is different on the server of course).
The kernel was built from commit eb7081409f94a9a8608593d0fb63a1aa3d6f95d8 and with the patch applied.