version: '3'
services:
  web:
    build:
      context: server
      dockerfile: Dockerfile-dev
    command: npm start # expecting bind port to 3000
  ...
  lt:
    build:
      context: lt # folder you created earlier with the Dockefile-lt inside
      dockerfile: Dockerfile-lt
    command: "usr/local/bin/lt --port 3000 --local-host web
    depends_on:
      - web