S allius/issue288 (#289)

* remove apostrophes from fmt strings

- thanks to @onkelenno for the suggestion

* improve the logger initializing

- don't overwrite the logging.ini settings if the
env variable LOG_LVL isn't well defined
- Thanks to @onkelenno for the idea to improve

* set default argument for LOG_LVL to INFO in docker files

* adapt unit test

app/Dockerfile

@@ -30,7 +30,7 @@ ARG SERVICE_NAME
 ARG VERSION
 ARG UID
 ARG GID
-ARG LOG_LVL
+ARG LOG_LVL=INFO
 ARG environment
 
 ENV SERVICE_NAME=$SERVICE_NAME