Fixed two issues in the 'Start a Go instance in your app' section

- See Issue 24 @ https://github.com/docker-library/golang
- Altered the explanation to indicate the ONBUILD trigger does a go install rather than go build via the go-wrapper script.
- Added some content on the CMD instruction.
- Changed order so it is build and run image so it matches the commands that follow.
I actually meant to leave in the paragraph that talks about the base image including `CMD ["app"]`, not putting it back in the `Dockerfile`.  This won't actually work in some cases (although it would work in a few), so we can't really do this here, but talking about this being the assumption in the "onbuild" images makes sense IMO (since we talk about `go install -v` being the assumption there, even though it's slightly more complicated than that).