Added a simplistic docker-compose example

[boomam]

I realized there wasn't one, after I made one for myself, so I thought I'd contribute the skeleton of one for others benefit. :-)

[mms-gianni]

Was about to open a PR. Thats the one I'm using :

version: "3.8"
services:
  tasks.md:
    build: .
    container_name: tasks.md
    image: baldissaramatheus/tasks.md
    ports:
      - "8080:8080"
    environment:
      - PUID=1000
      - PGID=1000
      - TITLE="My tasks board"
      - BASE_PATH=/tasks
    volumes:
      - ./backend/files:/api/files/
      - ./frontend/public/stylesheets:/usr/share/nginx/html/stylesheets/

[BaldissaraMatheus]

Thanks for the contribution, guys! I just merged the PR with some additional small changes