Feature request: support test subcommands?

[jmmills]

It would be nice to be able to breakout tests on a container into sub-commands, this would be useful for CI/CD integration v. Developer workflow.

Example, a python app that might need coverage reports generated in junit, but developers want console tests:

app:
  image: myorg/app
  build: Dockerfile
  test:
    default: nosetests --with-coverage --with-timer 
    ci: nosetests --with-xunit --with-coverage 

Thus, development workflow for testing would be done with captain test running the default target, and a CI server could call captain test ci to run a custom test command.