A typical cause is an application writing to a path that isn't mapped. Common mistakes are not using the same upper/lower case as in the mapping, or not using an absolute path /.
Note that this is referring to a setting within an application. For example, if you have transmission writing to Download, but your mapping has a container path of download, then it would write into the docker image because Download isn't mapped.
I came to this thread after seeing your more recent thread about filling cache. Probably you have several things wrong with the way you have setup your dockers that is causing multiple issues.