求教一个 docker 里 pip install 的问题

在 digitalocean sfo 机房的 ubuntu14.04 上配 docker 里的 django ，在系统里直接安装没任何问题：

pip install Django==1.8.13 --no-cache-dir --ignore-installed                                 
Collecting Django==1.8.13
  Downloading Django-1.8.13-py2.py3-none-any.whl (6.2MB)
    100% |################################| 6.2MB 6.8MB/s 
Installing collected packages: Django
Successfully installed Django-1.8.13

但是 docker 就不行了：

Step 6 : RUN pip install -r requirements.txt
 ---> Running in 6d2f65927b6f
Collecting Django==1.8.13 (from -r requirements.txt (line 1))
  Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after connection broken by 'ConnectTimeoutError(<pip._vendor.requests.packages.urllib3.con
nection.VerifiedHTTPSConnection object at 0x7fe9e3085f28>, 'Connection to pypi.python.org timed out. (connect timeout=15)')': /simple/django/
  Retrying (Retry(total=3, connect=None, read=None, redirect=None)) after connection broken by 'ConnectTimeoutError(<pip._vendor.requests.packages.urllib3.con
nection.VerifiedHTTPSConnection object at 0x7fe9e3085898>, 'Connection to pypi.python.org timed out. (connect timeout=15)')': /simple/django/
  Retrying (Retry(total=2, connect=None, read=None, redirect=None)) after connection broken by 'ConnectTimeoutError(<pip._vendor.requests.packages.urllib3.con
nection.VerifiedHTTPSConnection object at 0x7fe9e30858d0>, 'Connection to pypi.python.org timed out. (connect timeout=15)')': /simple/django/
  Retrying (Retry(total=1, connect=None, read=None, redirect=None)) after connection broken by 'ConnectTimeoutError(<pip._vendor.requests.packages.urllib3.con
nection.VerifiedHTTPSConnection object at 0x7fe9e30851d0>, 'Connection to pypi.python.org timed out. (connect timeout=15)')': /simple/django/
  Retrying (Retry(total=0, connect=None, read=None, redirect=None)) after connection broken by 'ConnectTimeoutError(<pip._vendor.requests.packages.urllib3.con
nection.VerifiedHTTPSConnection object at 0x7fe9e47c8160>, 'Connection to pypi.python.org timed out. (connect timeout=15)')': /simple/django/
  Could not find a version that satisfies the requirement Django==1.8.13 (from -r requirements.txt (line 1)) (from versions: )
No matching distribution found for Django==1.8.13 (from -r requirements.txt (line 1))
ERROR: Service 'web' failed to build: The command '/bin/sh -c pip install -r requirements.txt' returned a non-zero code: 1

Dockerfile 是这样：

FROM python:3.5
ENV PYTHONUNBUFFERED 1
RUN mkdir /usr/src/app
RUN mkdir /usr/log
WORKDIR /usr/src/app
ADD . /usr/src/app
RUN pip install -r requirements.txt

可能是什么地方的问题，是不是跟 python3.5 更新到 3.5.2 有关系？