Docker image for manage.py execution

As a manager/distributor of a project whose tasks are written to a `manage.py` file, I would like to be able to point my pals to a Docker image, which mounts the specified (or current) directory and whose entrypoint calls `manage` on a `manage.py` in that directory.

In this way, projects which do not have the same environmental requirements as `argcmdr`, (such as Python 3.6+), may require only that the environment supplies Docker, rather than Python 3.6 and/or virtualenv, on the host OS.