add app version output to version.txt

+ refactor main entry point
+ add debug logging in image extraction

Dockerfile

@@ -35,4 +35,7 @@ RUN pip install bjoern
 ARG app_version
 ENV SWINGMUSIC_APP_VERSION=$app_version
 
+# dump the app_version to the version.txt file
+RUN echo $app_version > version.txt
+
 ENTRYPOINT ["python", "run.py", "--host", "0.0.0.0", "--config", "/config"]