add a little documentation for docker pull

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
docker build: pull just latest if tag uspecified

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)