We will take a look at runC, a tool
for spawning and running containers according to the OCF specification.
You can read more about the Open Container Format specification at their website or in the announcement by Docker, Inc.
I use Fedora 22.
export GOPATH=$HOME/golang
mkdir -pv $GOPATH/src/github.com/opencontainers
cd $GOPATH/src/github.com/opencontainers
git clone https://github.com/opencontainers/runc
cd runc
make
sudo make install # forgive me
We have runC create a template container definition. We then create a directory called rootfs to keep our filesystem.
runc spec > config.json
mkdir rootfs
We pull the current CentOS image.
docker pull centos:7
We export said image to a tarball.
docker export $(docker create centos:7) > centos.tar
We then expand that tarball to the sub-directory rootfs, remove the tarball and chmod the directory to root.
tar -C rootfs -xf centos.tar
rm centos.tar
sudo chown root:root -R rootfs
Because UTS namespaces are not supported yet, we need to remove the hostname line.
sed -i '/hostname/d' config.json
We then make the rootfs writable (readonly: false). This command will remove the tabs before the readonly line. This is only cosmetical but easier than dealing with sed’s tab handling.
sed -i '/\"rootfs\"\,/!b;n;c\"readonly\"\:\ false' config.json
runC needs to run as root, yet root does not have /usr/bin/local in her $PATH so we call the binary with its full path.
/usr/local/bin/runc
We’re in.
sh-4.2#
While I was preparing this post, Georg Kunz of CloudGear wrote a very similar howto. And got featured in Docker Weekly. I think mine has enough difference and additional detail to publish it anyway. See also the official documentation, which at the time of writing is incomplete. Keep in mind that runC is still pre-alpha.