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.

docker-compose.yml

@@ -0,0 +1,27 @@
+version: '3.7'
+
+services:
+  backend:
+    build:
+      context: .
+      dockerfile: ./deploy/docker/Dockerfile
+    volumes:
+      - ./ssl:/opt/atat/atst/ssl
+      - ./config:/opt/atst/atst/config
+      - sockets:/var/run/uwsgi
+    environment:
+      PGHOST: host.docker.internal
+      REDIS_URI: "redis://host.docker.internal:6379"
+
+  frontend:
+    image: nginx:1.13-alpine
+    volumes:
+      - ./deploy/docker/sample.nginx.conf:/etc/nginx/conf.d/default.conf:ro
+      - sockets:/var/run/uwsgi
+    depends_on:
+      - backend
+    ports:
+      - 8080:80
+
+volumes:
+  sockets: