Refactor TestContainer

[tomuben]

Background

Today we have one class TestContainer which takes all arguments from client and then spawns test environment.
Independent of the kind of test database it passes all parameters to SpawnTestEnvironment.
SpawnTestEnvironment then distinguishes the kind of database and uses the internal docker db or an external provided db, see here

This causes some problems, as the interface is not well defined, see here.

Acceptance Criteria

The published interface by SpawnTestEnvironment should be clear. One idea is that the actual database is attached via DependencyInjection