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.
Well, that's one of the assumptions of the script (that it's run from the
root of the package you want to build, especially since that's where we
need to look for ".godir").  I suppose we should note that in the usage. :)