This PR changes the app user and group UID/GID to 1000 (default Linux user UID) instead of 10001. This fixes some permissions issues that can appear with Docker (for example, I got a few FirefoxSend.CRITICAL: uncaughtException Error: ENOENT: no such file or directory, unlink '/uploads/1-f3de0dbf117dd223' when uploading).
Tested and working.