Merge branch 'env-for-flask-shell' into 'master'

Add environment variables for flask shell

Closes #22

See merge request !11

README.md

 [![fsco.li/portal](./banner.png)](https://portal.fachschaft.cl.uni-heidelberg.de)
+
+# Development
+
+To run a flask shell inside the container:
+
+``` sh
+sudo docker exec -it portal-app-1 flask shell
+```

docker-compose.dev.yml

 services:
     app:
+        environment:
+            - FLASK_APP=portal
         build:
             context: .
             target: dev