Provide ability to use "/etc/aura/tank.yaml" as an optional configuration override

For Docker Compose deployments it is possible to override service configuration files in the container with given sample configurations.

To apply an override the contents of the host folder aura/config/services is mapped read-only into /etc/aura of the container:

    volumes:
      - ./../services/:/etc/aura/:ro

This has following effect:

1. Services read their configuration from /srv/config/${config-file} by default.
2. As soon there is a equally named config file under /etc/aura/${config-file} available, this overwrites the local configuration.

This way users can overwrite any container-internal configuration by simply copy-pasting another config into config/services on the host system.

To Do

Add logic that the Tank configuration file is (1) read from /etc/aura/tank.yaml. If such file is not existing, then (2) the default configuration file as part of the Docker image is used (e.g. located under /srv/config/tank.yaml).

The logic in e.g. engine and engine-api works the same and can be used for comparison.