Provide a smaller image (37 MB) based on Alpine Linux

[blueimp]

This is an alternative image based on Alpine Linux.
I propose to provide it via an additional tag for folks who value small image sizes.

The image size reported by docker images is only 37 MB compared to the 234 MB of the official image.

The list of changes compared to staging:

• Use alpine:edge as base image.
• Use tini as entrypoint.
• Clean up obsolete installation files.
• Fix missing ${DOCKER_HOST_IP} in the integration tests.

The tests ran successfully; I used the following command to run them:

docker run --rm -it \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v "$PWD:$PWD" \
  -w "$PWD" \
  -e DOCKER_HOST_IP=$(docker-machine ip) \
  blueimp/dind:1.9 \
  sh -c 'pip install websocket-client==0.32.0 six==1.9.0 && exec ./test.sh'

I've also set up an automated build of this alternative image on Docker Hub that you can try out with the following docker-compose.yml:

version: '2'
services:
  web:
    image: dockercloud/hello-world
  lb:
    image: blueimp/dockercloud-haproxy:1.2
    links:
      - web
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
    ports:
      - 80:80

Thanks for your consideration.

[tifayuki]

Thank you

[blueimp]

Thanks for merging!