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.
maybe it would be safer to always assume `/go/src/app`.

Any reason you use `rm -fr` + `ln -s` instead of `ln -sf`? are you worried there might be something else already there?

In that case wouldn't be safer to fail rather than deleting?