docker: separate frontend and backend network
docker: separate frontend and backend network

Even when the Docker host is not well configured (i.e. a link is not
required to communicate with containers), there is no way now that
nginx reaches other containers apart from uwsgi.

Signed-off-by: Marco Neumann <3829486b93ec44395f0b980424bae9b6fb07b7bc@crepererum.net>

spz-signup/spz-signup