Split dockerBase between builder and final stages so that final images can be slimmed down.

[ewilkins-csi]

Right now, both the image used to bundle the virtual environment during containerize-dependencies is exactly the same as the image used as the final packaging/runtime image. Because dev tools like gcc are often required during the build but not at runtime, final images can be much smaller if we allow building the virtual env with a more "full" base image and use a slimmed down version of that base image for the final image.

This does introduce some complexity for users as they will need to ensure that the value set for the builder stage is compatible with the value set for the final stage, but the old functionality can easily be used by simply providing the same value for both.