Add sdkContainerImage flag to (eventually) replace workerHarnessContainerImage

workerHarnessContainerImage (dataflow-only) is a legacy flag from a time where Dataflow packaged the worker and SDK together in on container. As we move towards portable world where remote runners will generally have a containerized SDK process, we wanted a new flag  that can be reused across runners. 

We want it to reflect that it's only the SDK and remove harness from the name, since "harness" is meaningless to most users ("what are we harnessing?").