Improvements to Docker files

From https://github.com/OpenDataServices/oc4ids-datastore-pipeline/pull/32

```
RUN apt-get update \
  && apt-get install --no-install-recommends -y libpq-dev gcc postgresql-client git \
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/*
```

`RUN pip install --no-cache-dir -e .`

saves HD space