Filling docker image is about the settings within an application that causes it to write to a path that isn't mapped.
If you use Transmission for example, and within the Transmission application you set it to downloading torrents to /download, but the container mapping is /Download, then that is going to write into the docker image, because Linux is case-sensitive and /download and /Download are different paths.
Or, if you tell it to write to download instead of /download, then that is going to be in the image also, since it is not an absolute path.
Taking them one at a time as suggested is the way to get this figured out.
Which dockers do you use?