Wait for database container to initialise, then run the migrations

Docker doesn't wait for all services in a container to become ready
before spinning up its dependencies. This gets round it by adding a
three second delay, then running the migrations.

For more information, see: docker/compose#374

docker-compose.yml

@@ -39,7 +39,7 @@ services:
 
   web:
     build: ./web
-    command: python manage.py runserver 0.0.0.0:8000
+    command: bash -c "sleep 3 && ./manage.py migrate && ./manage.py runserver 0.0.0.0:8000"
     volumes:
       - ./web:/code
     ports: