version: "3"

services:
  app:
    build:
      context: .
      dockerfile: docker/app/Dockerfile
    volumes:
      - my_folder:/var/www/html/vendor
      - .:/var/www/html

volumes:
  my_folder:

FROM composer AS php_builder
COPY . /app
#RUN composer install --no-dev
RUN mkdir -p vendor && echo "I'm dependency!" > vendor/dependencies.txt

FROM php:7.1-fpm
COPY --from=php_builder /app/vendor /var/www/html/vendor

shubuntu1@shubuntu1:~/test$ ls
docker  docker-compose.yaml  index.php
shubuntu1@shubuntu1:~/test$ docker-compose up -d
Creating network "test_default" with the default driver
Creating test_app_1 ... done
shubuntu1@shubuntu1:~/test$ docker exec -it test_app_1 /bin/bash
root@bf59d8684581:/var/www/html# ls
docker  docker-compose.yaml  index.php  vendor
root@bf59d8684581:/var/www/html# cat vendor/dependencies.txt
I'm dependency!