apt-get clean ineffective

In the Dockerfile you try to reduce the image size by cleaning the apt caches with

```
&& rm -rf /var/lib/apt/lists/* \
&& apt-get -qyy clean
```
the problem is that this only works when it is executed in the same `RUN` that also uses `apt-get update`, otherwise the data is already baked into a previous layer and all you achive is to record it as deleted which actually increases the layer size again.

See https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#apt-get