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.
Probably friendlier to users yes, but does blow up the number of scripts we copy around our repo, which is what I was trying to avoid.  Maybe we just have the script itself support being symlinked to `go-<cmd>` names that imply the command bit?