text: Title: Uwsgi using the wrong python version
  Body: Hello,
  
  I'm trying to update my app to Python 3.7 with new `tiangolo/uwsgi-nginx-flask:python3.7-alpine3.8` image. When launching the image with Docker compose, it seems like uwsgi is using Python 3.6.6 instead.
  
  ```
  Attaching to fakeimg
  fakeimg | Checking for script in /app/prestart.sh
  fakeimg | There is no script /app/prestart.sh
  fakeimg | /usr/lib/python2.7/site-packages/supervisor/options.py:461: UserWarning: Supervisord is running as root and it is searching for its configuration file in default locations (including its current working directory); you probably want to specify a "-c" argument specifying an absolute path to a configuration file for improved security.
  fakeimg |   'Supervisord is running as root and it is searching '
  fakeimg | 2019-02-19 12:48:32,061 CRIT Supervisor is running as root.  Privileges were not dropped because no user is specified in the config file.  If you intend to run as root, you can set user=root in the config file to avoid this message.
  fakeimg | 2019-02-19 12:48:32,061 INFO Included extra file "/etc/supervisor.d/supervisord.ini" during parsing
  fakeimg | 2019-02-19 12:48:32,076 INFO RPC interface 'supervisor' initialized
  fakeimg | 2019-02-19 12:48:32,076 CRIT Server 'unix_http_server' running without any HTTP authentication checking
  fakeimg | 2019-02-19 12:48:32,081 INFO supervisord started with pid 1
  fakeimg | 2019-02-19 12:48:33,083 INFO spawned: 'nginx' with pid 10
  fakeimg | 2019-02-19 12:48:33,086 INFO spawned: 'uwsgi' with pid 11
  fakeimg | [uWSGI] getting INI configuration from /app/uwsgi.ini
  fakeimg | [uWSGI] getting INI configuration from /etc/uwsgi/uwsgi.ini
  fakeimg | 
  fakeimg | ;uWSGI instance configuration
  fakeimg | [uwsgi]
  fakeimg | cheaper = 2
  fakeimg | processes = 16
  fakeimg | ini = /app/uwsgi.ini
  fakeimg | module = main
  fakeimg | callable = app
  fakeimg | wsgi-disable-file-wrapper = true
  fakeimg | ini = /etc/uwsgi/uwsgi.ini
  fakeimg | socket = /tmp/uwsgi.sock
  fakeimg | chown-socket = nginx:nginx
  fakeimg | chmod-socket = 664
  fakeimg | hook-master-start = unix_signal:15 gracefully_kill_them_all
  fakeimg | need-app = true
  fakeimg | die-on-term = true
  fakeimg | plugin = python3
  fakeimg | show-config = true
  fakeimg | ;end of configuration
  fakeimg | 
  fakeimg | *** Starting uWSGI 2.0.17 (64bit) on [Tue Feb 19 12:48:33 2019] ***
  fakeimg | compiled with version: 6.4.0 on 01 May 2018 17:28:25
  fakeimg | os: Linux-4.9.125-linuxkit #1 SMP Fri Sep 7 08:20:28 UTC 2018
  fakeimg | nodename: 786a77edca64
  fakeimg | machine: x86_64
  fakeimg | clock source: unix
  fakeimg | pcre jit disabled
  fakeimg | detected number of CPU cores: 4
  fakeimg | current working directory: /app
  fakeimg | detected binary path: /usr/sbin/uwsgi
  fakeimg | your memory page size is 4096 bytes
  fakeimg | detected max file descriptor number: 1048576
  fakeimg | lock engine: pthread robust mutexes
  fakeimg | thunder lock: disabled (you can enable it with --thunder-lock)
  fakeimg | uwsgi socket 0 bound to UNIX address /tmp/uwsgi.sock fd 3
  fakeimg | uWSGI running as root, you can use --uid/--gid/--chroot options
  fakeimg | *** WARNING: you are running uWSGI as root !!! (use the --uid flag) *** 
  fakeimg | Python version: 3.6.6 (default, Aug 24 2018, 05:04:18)  [GCC 6.4.0]
  fakeimg | *** Python threads support is disabled. You can enable it with --enable-threads ***
  fakeimg | Python main interpreter initialized at 0x56004504cfa0
  ```
  
  This is making my application crash, because every package has been installed for Python 3.7.2.
  The project is [here](https://github.com/Rydgel/Fake-images-please) if you need to view the Dockerfile.