Created by: wangkuiyi

This PR

1. removes paddle/scripts/docker/Dockerfile.m4 and paddle/scripts/docker/generate.sh, and
2. adds two Dockerfiles: Dockerfile for the GPU-only image and Dockerfile.gpu for the GPU image.

so that we can configure dockerhub.com to generate two official AVX-enabled images:

docker build -t paddle:cpu-latest -f paddle/scripts/docker/Dockerfile .
docker build -t paddle:gpu-latest -f paddle/scripts/docker/Dockerfile.gpu .

In order to support non-AVX computers, users need to manually build Docker images from source code, as I updated the document in this PR:

docker build --build-arg WITH_AVX=OFF -t paddle:cpu-noavx -f paddle/scripts/docker/Dockerfile .
docker build --build-arg WITH_AVX=OFF -t paddle:gpu-noavx -f paddle/scripts/docker/Dockerfile.gpu .

I think above manual work is reasonable because that

1. Dockerhub.com doesn't and seems will not support Docker arguments, and that
2. most computers (include VMs from all major cloud providers) produced after 2008 support AVX.