Add `hack/Dockerfile`, drop `COREOS_ASSEMBLER_GIT`

[cgwalters]

With the move to using even more Go, the workflow of just bind mounting the source code will break more and more.

(It has never worked for anything in mantle/)

Let's document a more robust workflow that ensures that the Go code is recompiled.

[dustymabe]

I'm sorry for not looking at this sooner. I'd like to not drop the existing workflow. I do have some local modifications to make things still work that I'd like to propose when I have time.

[nikita-dubrovskii]

@dustymabe do you have kind of PoC for your alternative?