Add heartbeat for haproxy; use global domain_root instead of auth_root.

Dockerfile

@@ -0,0 +1,5 @@
+FROM python:3.6-alpine
+ADD . /code
+WORKDIR /code
+RUN pip install -r requirements.txt
+CMD flask run -h $HOST

weather/__init__.py

@@ -9,7 +9,7 @@ from werkzeug.routing import FloatConverter
 
 app = Flask(__name__)
 
-auth_root = os.environ['AUTH_ROOT']
+domain_root = os.environ['DOMAIN_ROOT']
 ibm_root = os.environ['IBM_API_ROOT']
 
 # For some reason, the standard float converter rejects negative numbers
@@ -31,11 +31,15 @@ class HTTPPaymentRequired(HTTPException):
         super().__init__(description, response)
 
 
+@app.route('/heartbeat')
+def heartbeat():
+    return jsonify({'alive': True})
+
 @app.route('/api/v1/geocode/<float:latitude>/<float:longitude>/')
 def geocode(latitude, longitude):
     if not request.args.get('access_token'):
         abort(401)
-    user_req = requests.get(f"{auth_root}/api/v1/me",
+    user_req = requests.get(f"http://auth.{domain_root}/api/v1/me",
                             headers={'Authorization': f"Bearer {request.args['access_token']}"})
     user_req.raise_for_status()
     if not user_req.json()['is_subscribed']: