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.
Not bad, although I'm still not 100% sold on what makes that easier to
understand?  It's a lot less discoverable, and requires an extra line in
each Dockerfile to create those symlinks, so I don't see what this is
buying us, but I'm definitely happy to discuss further if we can come up
with a better justification for doing it. :)  (Since the actual
implementation would be pretty easy now.)