Add Dockerfile

[Sevenlive]

Regarding the Issue #25 i build a Dockerfile for using the prometheus exporter in docker. You just need to mount the Storage "/var/lib/varnish" from your varnish container as Read-Only and you're ready to go.

I would suggest adding a github Action as well, so everytime a new release is deployed, the github action would build a container and put it into the github Registry of this project.

version: '2'

services:
  varnish:
    image: varnish
    volumes:
      - varnish_tmpfs:/var/lib/varnish
      - "./default.vcl:/etc/varnish/default.vcl"
    ports:
      - 80:80

  varnishexporter:
    image: prometheus_varnish_exporter:master
    restart: always
    environment:
      VSM_NOPID: 1
    depends_on:
      - varnish
    ports:
      - 9131:9131
    volumes:
      - varnish_tmpfs:/var/lib/varnish/:ro