Proposed local docker solution for nipa

Hey all,

While working on my first proxy_ndp patch, I noticed that the checks
on patchworks are very useful (as I managed to get a lot of things
wrong), and for my second patch I wanted to run them locally first.

Then I saw the horror, that is the state of nipa's local capabilities,
with notes like this in the README: " `ingest_mdir.py` has not been
tested in a while so it's probably broken."

On the other hand, the wiki says, that you have to run the checks
locally, and you shouldn't waste machine time (and cause mailing list
spam) by running your checks on patchworks, so the situation is a bit
contradictory and not ideal.

I implemented a docker solution to run nipa locally, you can find my
branch at: https://github.com/nilcons-contrib/nipa/

And the main README file here:
https://github.com/nilcons-contrib/nipa/blame/docker/docker/README.rst

I'm sure, I will get a review from Kuba, but in the mean time, I also
wanted to ask the community to test this.  I'm interested in any
feedback, especially if the README was easy to understand, or if more
instructions are necessary.

Thanks,
Gergely