Created by: wangkuiyi

Fixes https://github.com/PaddlePaddle/Paddle/issues/625 Fixes https://github.com/PaddlePaddle/Paddle/issues/466

This PR does the following:

1. docker build does no longer run git clone but build Docker images from local source code. So that everyone can build Docker images from their recent work without the need to git push nor git tag.

2. PaddlePaddle version is not longer defined in CMakeLists.txt, but derived from Git tag (if there is one) or Git commit ID. Yes, we can go on running paddle version to print the version.

3. Reduce the number of Docker images for each version from 12 down to 4 -- AVX or GPU.

4. Every Docker image now contains documents.

5. Every Docker image now contains an OpenSSH server.

6. Update documents accordingly.

I'd thought that I can make each of above work in a small PR, but it is harder to divide-and-conquer than I imagined.