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.
Oh right, we pretend it's just "go install".  OK, I'm +1 on consistently
pretending we do "CMD ["app"]" too - we can separately explain the .godir
magic, and keep this simple.