Fix typo in docs for docker run -dns
runtime: correctly detect IPv4 forwarding

When memory cgroup is absent, there was not attempt to detect if IPv4
forwarding was enabled and therefore, docker was printing a warning
for each command spawning a new container. The test for IPv4
forwarding was guarded by the test for memory cgroup.
Adding a reference to ps -a

It was confusing to me as a first-time user that my docker attach command failed; I was expecting the container to run continuously, and when I couldn't attach to it, I assumed something was wrong. ps -a shows that the container still exists, which gives the user confidence to go to the next step.
Merge pull request #1497 from vincentbernat/fix/ipv4-forwarding-detection

runtime: correctly detect IPv4 forwarding
Merge pull request #1499 from seldo/master

* Documentation: Adding a reference to ps -a
Merge pull request #1494 from KSid/typo-docs-run-command

* Documentation: Fix typo in docs for docker run -dns
Do not overwrite ENV vars during 'docker run'.

- The HOME environmental variable is overwritten by 'docker run'.
  Only add the HOME env if it's not specified by the user.
- Apply the same trick to the PATH, container and HOSTNAME vars to
  prevent similar issues.
- Make sure that non-default ENV's still get through.

This fixes issue #1493