This commit adds a new `base_image` arg which can be used to specify the base image for building the app image, as used in the `FROM` command. Adding `base_image` allows for our `build-images` pipeline to specify `govuk-ruby-X.Y.Z` when packaging for production, pulling this base image from a private ECR repository; for local development, the base image will remain unchanged from the default as the `base_image` arg will not be overridden.