add dockerfile example

README.md

@@ -152,7 +152,13 @@ $ cntr attach boxbusy
 [root@55a93d71b53b:/var/lib/cntr]# cntr exec sh -c 'busybox | head -1'
 ```
 
-Also see Dockerfile.example on how to build a docker container with cntr.
+You can also use Dockerfile.example from this repo to build a docker container with cntr:
+
+``` console
+$ docker build -f Dockerfile.example . -t cntr
+# boxbusy here is the name of the target container to attach to
+$ docker run --pid=host --privileged=true -v /var/run/docker.sock:/var/run/docker.sock -ti --rm cntr attach boxbusy /bin/sh
+```
 
 ### LXD