ref(Dockerfiles): use tagged common image based on ubuntu:14.04

This PR introduces a minimal **deis/common** Docker image as the base to all our components. `make build` will now also build `deis/common:v0.12.0-dev` (or whatever our version.go resource contains), and we will need to set up the Docker Hub to build the same. 

Closes #1561, deis/base#7, deis/base#13, deis/base#14.
See, that's the thing: we don't ever want to use ":latest" again, or we're right back in the **deis/base** problem where we break previous users if we refactor the base...err, common image.

But maybe tagging it separately for 0.12.0-dev, then 0.12.0, then 0.13.0-dev, etc. is too much churn for how rarely this is likely to change? It gives us the versioning control we wanted, which is good.

(Or we just chuck out deis/base | deis/common altogether, which I still prefer.)