Convenient configuration

[HenningCash]

ATM we use a configuration based on Spring's application.yml. It works, but it's a bit inconvenient. Configuring e.g. postgres relies on bare spring properties. There should be some form of abstraction via codefreak. properties.

Also, the documentation misses information on how to apply configuration to the Docker image. This is currently done by mounting the application.yml into a location known by Spring.