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.
1. correct
2. go-wrapper/.godir explanation sounds like a good plan, maybe @proppy or @tianon could write one up?
3. you can just `git checkout` each of those files so they go back to the previous state (next time you can just run `./update.sh golang` to just do the one)