To be able to export a container, first you have to create one:

docker create --name cont1 alpine sh

Now you are ready to export it into a tar file: alpine.tar

docker export cont1 > alpine.tar

It can take some time, be patient...

You can remove the container: docker rm -f cont1

To create the rootfs dir, just untar alpine.tar into a directory named roots: mkdir rootfs; tar -xf alpine.tar -C rootfs

With the Rootfs ready you need to generate 2 spec files. runC can create a default file based on the rootfs directory using:

runc spec

You are ready to start the runC container:

sudo runc start

Because we specified the export container to start sh you should be in a alpine prompt. For example you'll now have access to apk, Alpine's package manager.

apk --version