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.
Safer, but a lot less interesting! :)  (ie, I always try to minimize
external assumptions that don't actually matter or don't provide value)

I definitely agree with failing instead of deleting though, not sure what I
was thinking here.  Just using "ln -sf" won't do at all what we meant if
the target already exists as a directory, for example, so I'll update to
just fail in the case that it already exists and isn't a symlink already.