Update Dockerfile to use environment vars
Update Dockerfile to use environment vars

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

jwhonce/dockramp,jlhawn/dockramp,beni55/dockramp,ankushagarwal/dockramp,jlhawn/dockramp,jimt/dockramp,beni55/dockramp,jwhonce/dockramp,saromanov/dockramp,ankushagarwal/dockramp,ankushagarwal/dockramp,saromanov/dockramp,jlhawn/dockramp,jimt/dockramp