Preserve correct media directory in Docker.
Preserve correct media directory in Docker.

[`MEDIA_ROOT`](https://github.com/kobotoolbox/kobocat/blob/master/onadata/settings/kc_environ.py#L70) location changed since [change to `PROJECT_ROOT` in `4b2b30`](https://github.com/kobotoolbox/kobocat/commit/4b2b30bd4c93c967bd219c66fb81a6816f24d9f6#diff-fe503402bfe64d2b53a020632d2aacb7R28).

Inspired by kobotoolbox/kobo-docker#68.
kobotoolbox/kobocat,kobotoolbox/kobocat,kobotoolbox/kobocat,kobotoolbox/kobocat