Node Docker Project

.env.service

RDS_HOSTNAME=db
RDS_USERNAME=root
RDS_PASSWORD=some_pass
NODE_PATH=/app
MESSAGE_SERVICE_USERNAME=guest
MESSAGE_SERVICE_PASSWORD=guest
MESSAGE_SERVICE_PORT=5672
MESSAGE_SERVICE_VHOST=/
MESSAGE_SERVICE_HOST=rabbit

docker-compose.yml

db:
  image: mysql:5.6
  ports:
    - "3306:3306"
  env_file:
    - '.env.db'

rabbit:
  image: rabbitmq:3.6-management
  env_file:
    - '.env.rabbit'
  ports:
    - "5672:5672"
    - "15672:15672"

service:
  build: .
  volumes:
    - .:/app
  ports:
    - "8888:8888"
  links:
    - db
    - rabbit
  env_file:
    - '.env.service'

Dockerfile

FROM node:5.5

RUN mkdir /app
WORKDIR /app

ADD . /app/

RUN npm install

What does this sequence of RUN/WORKDIR/ADD do? Generates a directory, cd to it, but then the ADD . /app/? Is app just used for the volume mount?