version: "2"

services:
  rocketchat:
    image: rocketchat/rocket.chat:0.73.1
    ports:
      - 127.0.0.1:3000:3000
    restart: always
    environment:
      - PORT=3000
      - ROOT_URL=http://localhost:3000
      - MONGO_URL=mongodb://mongo:27017/rocketchat

    depends_on:
      - mongo
    links:
      - mongo:mongo
      - mongo:secure.gravatar.com

  mongo:
    image: mongo:3.3
    restart: always
    command: mongod --smallfiles
    volumes:
     - ./data/db:/data/db

version: "3"

services:
  rocketchat:
    image: rocketchat/rocket.chat:0.73.1
    ports:
      - 127.0.0.1:3000:3000
    restart: always
    environment:
      - PORT=3000
      - ROOT_URL=http://localhost:3000
      - MONGO_URL=mongodb://mongo:27017/rocketchat

    depends_on:
      - mongo
    links:
      - mongo:mongo
      - mongo:secure.gravatar.com

  mongo:
    image: mongo:3.3
    restart: always
    command: mongod --smallfiles
    volumes:
     - ./data/db:/data/db

Update docker-compose version to 3
Update docker-compose version to 3
