Add documentation for system containerd

Add an example script showing how you can run an nginx container.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

docs/containerd.md

@@ -0,0 +1,30 @@
+## Using the system containerd
+
+Here is a simple example script that will run a container using the system containerd.
+
+You should run it from `/var` as the root filesystem is in RAM, and will use up memory.
+
+```bash
+#!/bin/sh
+
+NAME=nginx
+VERSION=latest
+
+docker pull ${NAME}:${VERSION}
+CONTAINER=$(docker create --net=host --security-opt apparmor=unconfined --oom-score-adj=-500 -v /var/log/nginx:/var/log/nginx -v /var/cache/nginx:/var/cache/nginx -v /var/run:/var/run ${NAME}:${VERSION})
+docker run -v ${PWD}:/conf -v /var/run/docker.sock:/var/run/docker.sock --rm jess/riddler -f -bundle /conf ${CONTAINER}
+rm -rf rootfs && mkdir rootfs
+docker export ${CONTAINER} | tar -C rootfs -xf -
+docker rm ${CONTAINER}
+
+mkdir -p /var/log/nginx /var/cache/nginx
+
+containerd-ctr containers start ${NAME} .
+containerd-ctr containers
+```
+
+For debugging it helps to run `containerd-ctr containers start --attach ${NAME} .` It may
+well turn out that you need to create directories that are empty volumes in docker.
+
+For production, you will want to create the `config.json` offline and bundle it in with your
+intii script, but you can create the rootfs online.