Doc improve: docker-compose example

[hxzhao527]

The docker-compose example can't exec server command after server startup.

According to the docker-doc, we need add stdin_open: true,

version: '3'
services:
  terraria:
    image: beardedio/terraria:vanilla-1.4.0.4
    ports:
      - '7777:7777'
    environment:
      - world=The_Common_Tropic.wld
    volumes:
      - ./config:/config
    tty: true
    stdin_open: true

Then, we can use docker attach to exec server-command.

[lpeabody]

Yup, this should be updated in the README.md for sure. I ended up having to do the same thing, but only after I read this issue #4 and noticed they added the stdin_open: true option to their service configuration.

[hskrtich]

I have added this to the readme, Thanks!