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 agree that adding a section about `go-wrapper` would be good, and a lot of it could probably be borrowed directly from the `go-wrapper` usage text.  Definitely good fodder for a new PR though. :smile: