Fix non-existing postgres 'engine' database and wrong container name in docs

[ulikoehler]

When trying to run this application

a) while running
docker-compose run engine python manage.py migrate
I encountered this error message:
ERROR: No such service: engine

since the Django container is named web in docker-compose.yml and not engine.
This is fixed by renaming the relevant commands in the app README.

b) Startup errors (in the docker-compose logs) relating to the postgres database engine not existing.

The reason for this is that the postgres container creates a DB named postgres by default. This pull request resolves that issue by adding POSTGRES_DB: engine to docker-compose.yml. Please note that I did not add those to all other compose configs like docker-compose.prod.yml.

Note: If you have already started up the container, postgres will not automatically re-initialize. You need to delete the pgdata volume using docker volume rm app_pgdata but note that deleting the volume WILL result in loss of all data already being in the database

[ayoubeddafali]

LGTM!