Dockerfile for Development/demo

[nhoening]

A Dockerfile to run a FlexMeasures Docker container, installing from local code or from Pypi (or make both possible).

This can be quite useful as a quickstart to see FlexMeasures in action. To make this use case possible, we also should provide a toy database script.

This issue is also useful as a starting point for using FlexMeasures on Docker in production (e.g. in Kubernetes). So keep that in mind and let's put follow-up tickets for that in this same project.