Support run eth-netstats with docker

[jamesdave]

I add Dockerfile so it easy to deploy with docker's user

It can be used with compose file:

version: '3'
services:
  netstats:
    ports:
     - 3000:3000
    environment:
     - WS_SECRET=SgePVom6GQLv0nYnil9O3qAnCM5K2p6T
    build: .