Refactor the runner so that executors provide the connection to the step-runner

Problem

The implementation of steps for the Docker executor is not able to be extended easily to other executors.

Each executor is responsible for setting up the runner environment, and as such, could return a connection to the step-runner.

Proposal

Extend the Executor interface by adding a method Connect() io.ReadWriteCloser. Refactor the Docker executor to use this approach.