CI: Add container build

[zmerlynn]

Add a simple Dockerfile that can build kops, and a short README on how
to build it. This generates a 425M image, which is about as tight as I
could get it in a short amount of work without just busting apart the
golang:1.6-alpine image to compress the Go layer itself. It's good
enough for what I'm planning to use it for: CI builds.

Along the way: Fix-up .gitignore to include more from k8s/.gitignore
(include editor ones, and terraform output dir).

[justinsb]

LGTM - thanks.

Not sure I particularly care about image size - we can always just create a statically linked version of kops if needed and run with no OS at all. But I'm glad you do :-)

[zmerlynn]

Enh, the image size without Alpine and w/o pruning is into a whopping ~3GB or so. At that point I cared. Then it just became a game. :)

[zmerlynn]

But actually, having played with it on another project, the musl linked binaries are fine (versus static), so if we really cared about it, the next easiest step is just to extract the binaries and /model/ out of this build image and slap them into an alpine:3.4 image, which accomplishes the layer compression. The image in this PR is almost the same resulting filesystem, though, and eventually it looks like we'll have squash: moby/moby#22641