Talk up the 1.4 change to initialise volumes at  time

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

this probably should have been in #9089
This was a little confusing to me. Why is the user making this container? To demonstrate shared volumes? Need to add some context, I think. 
We can also clarify to: 
While this container doesn't run an application, it reuses the `training/postgres` image so that all containers are using layers in common, which saves disk space.