Single Dockerfile for building ATAT.

Dockerfile is now a single multi-stage build that relies on a Python 3.7
base image.

Notes:
- This builds uWSGI with a `pip install` because the Alpine vendored
  uWSGI is built against Python 3.6.
- Adds a docker-compose file that can be used for testing that the build
  works. It is not usable for development purposes because it creates a
  static copy of the application.

uwsgi.ini

@@ -0,0 +1,9 @@
+[uwsgi]
+callable = app
+module = app
+socket = /var/run/uwsgi/uwsgi.socket
+plugin = python3
+plugin = logfile
+virtualenv = /opt/atat/atst/.venv
+chmod-socket = 666
+chown-socket = atst:atat