docker: add 'docker exec'

cheat/cheatsheets/docker

@@ -4,6 +4,9 @@ docker -d
 # start a container with an interactive shell
 docker run -ti <image_name> /bin/bash
 
+# "shell" into a running container (docker-1.3+)
+docker exec -ti <container_name> bash
+
 # inspect a running container
 docker inspect <container_name> (or <container_id>)