[RFC] Construct swift versions using environment variables

[swizzlr]

Less duplication, it seems. cf. https://github.com/OliverLetterer/swift-docker/blob/master/Dockerfile

[hamin]

this makes perfect sense @OliverLetterer you wanna do this? If not i can submit PR

[OliverLetterer]

Sorry for bailing out yesterday without notice, had to really get some sleep. Would have loved to submit a pull request for this but I see there is already #11 :)

[swizzlr]

Closed by #14