refactor Dockerfile out of generic openeo-aggregator repo

[soxofaan]

related to #117

This repo has a dockerfile, which was originally specific to the openEO Platform deploy (e.g. openEO Platform configs are baked in).
With the aim of making openeo-aggregator more generic, this doesn't make sense anymore.

openeo-aggregator should just provide for building wheels (and possibly publish on PyPI), further packaging (e.g. with Docker) is to be handled in other pipelines