Put gunicorn in AS

[gustavofuhr]

As of right now we're running using python3 run_api.py in supervisor,
which uses the Flask dev server.

It's good for develop since it has live reload, but I made some tests
by using multiple workers in gunicorn and the listing of images is
much better (obviously). Did not see any increase of performance for
a single worker, btw.

To do that, it seems that we should have a Dockerfile for deploy, even
if local.