Endpoint Support for Container Service

_As a site administrator I want to be able to specify how containers are obtained so I can run containerized functions_

# Assumptions
1. Should the endpoint or the executor have the knowledge of what sort of containers are used?

No files, tests, or entry points are named. Start by resolving whether the endpoint or executor should own knowledge of how containers are obtained, then trace the relevant endpoint and executor configuration; done requires a defined container acquisition flow that supports containerized functions.

Written by the indexing model from the issue text.