Add simple "go-wrapper" script for building and running, especially to accommodate ".godir" support

Fixes #4
Fixes #9

Also, `update.sh` will make sure it's properly copied into each version subfolder, like we've done with other official images.
So let's go for the wrapper for simplicity.

I'm not a fan of the 'wrapper' name, I'd prefer something more friendly like `go-tool` or `go-builder`. But since it's not directly surfaced to the user of the :onbuild version I can live with it as an implementation detail.