- django is now launch in the foregroud, and its stdout/stderr is now
  the container stdout/stderr

- nginx is now run as a background process, without supervision. A
  HEALTHCHECK CMD is added to monitor the status of nginx (so as to
  a nagios alert in case an hypothetical nginx crash)

- add ALLGO_HTTP_SERVER to select the http server. Default is
 'gunicorn', docker-compose.yml sets it to 'django' for development.