Merging host and container content in docker

I have a docker sample project that scope contains content on startup. Are there any methods for POSTING the host and container contents on a volume, but keep the host contents in the container?

this is my compose.yml

services:
  db:
    image: mysql
    environment:
      MYSQL_DATABASE: ninja
      MYSQL_ROOT_PASSWORD: pwd
  app:
    image: invoiceninja/invoiceninja
    volumes:
      - ./install-wipay.sh:/install-wipay.sh
      - ./composer.json:/var/www/app/composer.json
      # would like to merge and persist this volume
      - ~/Documents/Git/heroku/invoiceninja/app/:/var/www/app
      - ~/Documents/Git/omnipay-wipay/:/var/repo/omnipay-wipay/:ro
      - ./.env:/var/www/app/.env:ro
      - ./storage:/var/www/app/docker-backup-storage:rw
      - ./logo:/var/www/app/docker-backup-public/logo/
    links:
      - db:mysql
    env_file: .env
  web:
    image: nginx
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro