Updates the CI to use the latest Swift 4.2.3 release, and the official Swift Docker images.
Using the official Docker images can speed up CI build times as the build does not need manually installing from swift.org, and system dependencies are already set up.
Updates the CI to use the latest Swift 4.2.3 release, and the official Swift Docker images. Using the official Docker images can speed up CI build times as the build does not need manually installing from swift.org, and system dependencies are already set up.