support UBUNTU MIRROR and modify doc

doc/getstarted/build_and_install/docker_install_en.rst

@@ -142,6 +142,22 @@ to install CUDA driver and let Docker knows about it:
     export DEVICES=$(\ls /dev/nvidia* | xargs -I{} echo '--device {}:{}')
     docker run ${CUDA_SO} ${DEVICES} -it paddledev/paddle:gpu-latest
 
+    
+UBUNTU MIRROR
+-------------
+
+Building Paddle Docker image hits some wrong with apt-get update, you
+can use other UBUNTU MIRROR instead of the default
+
+.. code-block:: bash
+
+    cd ~
+    git clone https://github.com/PaddlePaddle/Paddle.git
+    cd Paddle
+    git submodule update --init --recursive
+    docker build --build-arg UBUNTU_MIRROR="http://mirrors.163.com" -t paddle:cpu-avx -f paddle/scripts/docker/Dockerfile .
+    docker build --build-arg UBUNTU_MIRROR="http://mirrors.163.com" -t paddle:gpu-avx -f paddle/scripts/docker/Dockerfile.gpu .
+
 
 Non-AVX Images
 --------------

paddle/scripts/docker/Dockerfile

@@ -2,6 +2,8 @@ FROM ubuntu:14.04
 MAINTAINER PaddlePaddle Authors <paddle-dev@baidu.com>
 
 ARG DEBIAN_FRONTEND=noninteractive
+ARG UBUNTU_MIRROR
+RUN /bin/bash -c 'if [[ -n ${UBUNTU_MIRROR} ]]; then sed -i 's#http://archive.ubuntu.com#${UBUNTU_MIRROR}#g' /etc/apt/sources.list; fi'
 RUN apt-get update \
     && apt-get install -y cmake libprotobuf-dev protobuf-compiler git \
     libgoogle-glog-dev libgflags-dev libgtest-dev \

paddle/scripts/docker/Dockerfile.gpu

@@ -2,6 +2,8 @@ FROM nvidia/cuda:7.5-cudnn5-devel-ubuntu14.04
 MAINTAINER PaddlePaddle Authors <paddle-dev@baidu.com>
 
 ARG DEBIAN_FRONTEND=noninteractive
+ARG UBUNTU_MIRROR
+RUN /bin/bash -c 'if [[ -n ${UBUNTU_MIRROR} ]]; then sed -i 's#http://archive.ubuntu.com#${UBUNTU_MIRROR}#g' /etc/apt/sources.list; fi'
 RUN apt-get update \
     && apt-get install -y cmake libprotobuf-dev protobuf-compiler git \
     libgoogle-glog-dev libgflags-dev libgtest-dev \