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.
This is not true.  The `CMD` is `["go-wrapper", "run"]`, which then runs whatever your app is, independent of its name (since the normal output of go build is dependent on the name of the app being built).