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.
This is actually technically a Docker limitation, but that's a digression. :smile:

So, my problem with that is that it makes them seem more official than they really are, ie, `go-install` will now be confusingly similar to `go install` but ever so slightly different, which is what I was trying to avoid by keeping everything in the `go-wrapper` script directly (so it's clearly separated).