How to properly expose Marathon Applications

[mrocklin]

The current solution in this repository is only really useful for testing. It uses the agent's standard environment. Most people I've spoken to use docker containers instead. What is the right way to expose the idiomatic use of docker containers for this project without tying ourselves to one particular solution favored by one particular company or lab? What is a good and generic way to help people specify docker containers of their choice while still ensuring that dask-worker is installed and runs?

cc @kszucs @hussainsultan