Leaner Docker image through the use of multi-stage builds

[brandur-stripe]

In #50 I was able to successfully push an image to Docker Hub, but quite
a large one -- weighing in at 91 MB, which is pretty heavy for this
small program. It's big because it includes the entire Go build system.

Here we lean out the final image by switching Dockerfile to use a
multi-phase build strategy. This creates one image initially based off
of a Go image which is used to build the final release program, then
creates a second image based on the lightweight Alpine Linux and copies
over the release binary.

This has the effect of reducing the final image's size from 91 MB to a
lean 13.7 MB (that's Alpine Linux + stripe-mock), which is faster/cheaper
for users to pull.

A smaller optimization is to add a .dockerignore file which prevents
us from having to copy a number of extraneous files into the Docker
daemon for the initial build. This will help nominally to speed things
up.

One slight loss is that I've removed testing within the Go image from
.travis.yml because we no longer have a Go runtime with which to run
go test ./.... I think this is okay because we're already running
tests elsewhere in the Travis build, and it's unlikely that running them
in a Docker container is going to surface anything extra.

Here's me running the image locally:

$ docker run -d --name stripe-mock-container -p 12112:12111 stripemock/stripe-mock
bc71abebe5f6d80a9af2d1d53c86777d8c50b2e9a8d4de4a8b25d66c32a5d44b

$ docker ps
CONTAINER ID        IMAGE                    COMMAND                  CREATED             STATUS              PORTS                    NAMES
bc71abebe5f6        stripemock/stripe-mock   "/bin/sh -c /stripe-…"   26 seconds ago      Up 28 seconds       0.0.0.0:12112->12111/tcp stripe-mock-container

And a successful cURL:

$ curl http://localhost:12112
{
  "error": {
    "message": "Please authenticate by specifying an `Authorization` header with any valid looking testmode secret API key. For example, `Authorization: Bearer sk_test_123`. Authorization was ''.",
    "type": "invalid_request_error"
  }
}

r? @ob-stripe
cc @stripe/api-libraries

[ob-stripe]

This is awesome.

[brandur-stripe]

And actually, Docker Hub reports that the compressed side is 6 MB even! Big improvement.