Add explicit info explaining why this Dockerfile doesn't 'just build'

[SvenDowideit]

closes #36 - yes, its not really needed, but it would have helped in some of my choices.

[vito]

Sorry for the confusion, we actually just need to clean this repo up and reconcile it with concourse/concourse - we actually use a different Dockerfile for building the image now:

https://github.com/concourse/concourse/blob/master/ci/dockerfiles/rc/Dockerfile

We should probably move it back over here and start using this repo for building instead.

[vito]

I've added a section to the README explaining this a bit: https://github.com/concourse/concourse-docker#building-concourseconcourse

Let me know if there's more that should be done! Closing this as it no longer applies (the Dockerfile is a bit different and only depends on a linux-rc now).

[yuzs2]

I've added a section to the README explaining this a bit: https://github.com/concourse/concourse-docker#building-concourseconcourse

Let me know if there's more that should be done! Closing this as it no longer applies (the Dockerfile is a bit different and only depends on a linux-rc now).

how can we get the "linux-rc"?