Swift 4.2 development snapshots

[florianreinhart]

It would be great to have Swift Docker images for Swift 4.2 development snapshots. I think it's good enough to update them manually every or every other week to the latest Swift 4.2 snapshot.

I could update the 4.2 image regularly in this repo. However, I don't know what's needed to merge them into the official Docker images repo.

[florianreinhart]

Swift 4.2 is now in "final convergence" and convergence snapshots are available for download.

In addition, Swift 4.2 now supports Ubuntu 18.04. I think we should use Ubuntu 18.04 as base image for Swift 4.2 or at least provide a Ubuntu 18.04 variant.

@hamin @swizzlr What do you think is best? I will then create a PR.