Improve slow shutdown

[mikehaertl]

I found, that the image has quite a long shutdown times for me of > 10 seconds. It's probably a similar issue as described here: clue/docker-adminer#17:

Current form of CMD does not passes signals to the supervisord and container can not be stopped via standard docker stop command. After a timeout, container is killed by Docker.

I know that supervisord is not used here, but maybe the same problem exists for php -S.

[TimWolla]

Use the Docker 1.13 --init flag, please. See also the discussion for my other official image spiped: docker-library/docs#814

I'm going to update the README files accordingly.

[TimWolla]

Correcting myself: I was able to fix the issue, without adding any additional executables. So I went for that one. I can recommend --init nonetheless.