docker - How to mount a single file in a volume

Question

I am trying to dockerize a PHP application. In the dockerfile, I download the archive, extract it, etc.

Everything works fine. However, if a new version gets released and I update the dockerfile, I have to reinstall the application, because the config.php gets overwritten.

So I thought I can mount the file as a volume, like I do with the database.

I tried it two ways, with a volume and a direct path.

docker-compose:

version: '2'
services:
  app:
    build: src
    ports:
      - "8080:80"
    depends_on:
      - mysql
    volumes:
      -  app-conf:/var/www/html/upload
      -  app-conf:/var/www/html/config.php
    environment:
      DB_TYPE: mysql
      DB_MANAGER: MysqlManager

  mysql:
    image: mysql:5.6
    container_name: mysql
    volumes:
      - mysqldata:/var/lib/mysql
    ports:
      - 3306:3306
    environment:
      MYSQL_ROOT_PASSWORD:
      MYSQL_DATABASE:
      MYSQL_USER:
      MYSQL_PASSWORD:

volumes:
  mysqldata:
  app-conf:

Which results in the error:

And I tried it with a given path, as a mounted volume.

/src/docker/myapp/upload:/var/www/html/upload
/src/docker/myapp/upload:/var/www/html/config.php

However, both ways are not working. With the mounted volume, I see that upload gets created.

But then it fails with:

/var/www/html/config.php" caused "not a directory"""

If I try it with

/src/docker/myapp/upload/config.php:/var/www/html/config.php

Docker creates the upload folder and then a config.php folder. Not a file.

Or is there another way to persist the config?

Answer 1

TL;DR/Notice:

If you experience a directory being created in place of the file you are trying to mount, you have probably failed to supply a valid and absolute path. This is a common mistake with a silent and confusing failure mode.

File volumes are done this way in docker (absolute path example (can use env variables), and you need to mention the file name) :

    volumes:
      - /src/docker/myapp/upload:/var/www/html/upload
      - /src/docker/myapp/upload/config.php:/var/www/html/config.php

You can also do:

    volumes:
      - ${PWD}/upload:/var/www/html/upload
      - ${PWD}/upload/config.php:/var/www/html/config.php

If you fire the docker-compose from /src/docker/myapp folder