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.
Would you mind adding back the Oxford Comma here? :smile:

https://github.com/docker/docker/blob/a29015a49a27d67b432d294380ab0e7f14f17f0b/docs/sources/contributing/docs_style-guide.md#oxford-comma
https://github.com/docker/docker/pull/8500/files#diff-041fd0362c7008ab638faef16a04c61aR155