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, I liked "go-wrapper" because it's very obviously not a "Go upstream"
tool/script (and I don't really want our users going to Go upstream
complaining about "go-wrapper" doing strange things for them).  Other than
that, I'm open to suggestion (but that's what I don't like about "go-tool"
or "go-builder" or similar).