clean up the current container API

From robertwb
As part of this project, I propose we look at and clean up the current container API before we "release" it as public and stable. IIRC, we currently provide the worker arguments through a combination of (1) environment variables (2) command line parameters to docker and (3) via the provisioning API. It would be good to have a more principled approach to specifying arguments (either all the same way, or if they vary, good reason for doing so rather than by historical accident).