DOCKER_VERSION and docker-version havn't been implemented.

So far, it looks like the declarations are not used, and so its safer not to
confuse people into thinking they do something.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

@tianon when was the last time someone made sure that our main `Dockerfile` worked on Docker 0.6.1, and are we really intending to send that message?

@jfrazelle are you suggesting that your Dockerfiles cannot be used with older Docker daemons? - and which is it :) (I know where these came from)

@me - seriously, do you really thing Docker 0.3 is something we want to bring up in the docs?

On a serious note - is this INSTRUCTION being worked on, in which case I'll reduce the scope to the documentation examples, where its distracting, or can we move on?

@crosbymichael feel free to yell at me about busting the cache - maybe we can use the golang:1.3-onbuild image - its only 1.5GB big >:+