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.
Just because I love the logical separation - we get a "download all the deps" step separate from a "actually build and install the software" step (and can see which one is the real slowdown very obviously).  If you don't agree that it's valuable, I'm not opposed to removing it.