Improve docs to include simple 2-liner to run Tox using Docker.

[wjlow]

No description provided.

[MarSoft]

I think we should not mount directory to /app but rather mount it to /src. The /docker-entrypoint.sh script copies everything except for dot directories from /src to /app, then it chowns /app and runs tox over it. We don't want to chown in our real filesytstem (and sometimes we won't be able to do it anyway), so we want to mount our source to /src instead.