Container caching using docker volumes

Added

• swift docker test --seed-build-foler - Copy the current .build folder to the container volume
• swift docker test --clean - Delete and re create the .build folder in the container volume

Changed

• swift docker test no longer builds an image to run the tests. Instead it:
  • uses a bind mount to sync the source code between the host and the container
  • create/re-uses a docker volume for the .build folder