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.
I wonder if separate script would make this friendlier, when calling it from the non-onbuild image:
```
go-run
go-get
go-build
``