docker-stack.yml should take value of DJANGO_SECRET_KEY from env
docker-stack.yml should take value of DJANGO_SECRET_KEY from env

We should override SECRET_KEY in production. We'll do it by setting an
env variable in Travis.

kinaklub/next.filmfest.by,kinaklub/next.filmfest.by,kinaklub/next.filmfest.by,nott/next.filmfest.by,kinaklub/next.filmfest.by,nott/next.filmfest.by,nott/next.filmfest.by,nott/next.filmfest.by