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.
My problem with that is that it was never clear to me (as a layman "go
tool" user) that a raw "go get" is really "download + build + install", and
so I hate perpetuating that verbiage choice.