Hi,

When building the example Front-Proxy the Step 6 in the Dockerfile-service (RUN pip3 install -q Flask==0.11.1) fails:

Step 6/11 : RUN pip3 install -q Flask==0.11.1
---> Running in b66bfce3d782
*** Error compiling '/tmp/pip_build_root/Jinja2/jinja2/asyncfilters.py'...
File "/tmp/pip_build_root/Jinja2/jinja2/asyncfilters.py", line 7
async def auto_to_seq(value):
^
SyntaxError: invalid syntax

*** Error compiling '/tmp/pip_build_root/Jinja2/jinja2/asyncsupport.py'...
File "/tmp/pip_build_root/Jinja2/jinja2/asyncsupport.py", line 22
async def concat_async(async_gen):
^
SyntaxError: invalid syntax

This error was reproduced using Docker for Windows running on Windows 10 and also on Ubuntu 16.04 LTS server.

This issue was first mentioned at #1597 but was Closed and Merged with #1617