Added warning if in docker and data dir isn't mounted

Had to remove the VOLUME definition in the dockerfile for this, otherwise the data dir would always be mounted by an anonymous volume if the user didn't mount it correctly, so the python script wouldn't be able to detect it.

Dockerfile

@@ -66,6 +66,5 @@ ENV BUILD_DATE=${BUILD_DATE}
 
 # Run command
 EXPOSE 8000
-VOLUME ["/home/app/.local/share/spoolman"]
 ENTRYPOINT ["uvicorn", "spoolman.main:app"]
 CMD ["--host", "0.0.0.0", "--port", "8000"]