Introduce a Dockerfile based on tiangolo/uwsgi-nginx. That image
requires the app to be placed at /app and runs uwsgi with options from
/app/uwsgi.ini. The image also executes /app/prestart.sh which we are
currently using to perform database migrations and static collection.

Thi commit also provides a convenience script (start-docker.sh) to start
the docker image.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>