various changes to make the image more focused and concise

Hi Thierry,

I made some changes to make your docker image - from my perspective - more focused to the task. Further php dependencies can easily be added by creating a very small Dockerfile inheriting from this. I don't wanna comment on everything, just a few remarks:

*   debian instead of ubuntu, because it's dockers choice for most base images, also it's really tiny
*   COPY > ADD (see bestpractices)
*   The have the concept of an entrypoint which allows us to either run nginx or any other command given by the user. We don't need ssh anymore.

Are you interested in merging this? Otherwise I'd just maintain this independently.

Tim