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.
Didn't include the literal CMD to remain consistent with the explanation of the ONBUILD triggers which also use go-wrapper.

Does the COPY instruction not copy the application's directory content to the /go/src/app directory, which the "ONBUILD RUN go-wrapper install" will install, creating the /go/bin/app executable based on the app directory name ?

Should I remove the CMD explanation ? Just thought the Dockerfile looked a little odd without an explanation.