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

Answer 2

I had been suffering from a similar issue. I was trying to import my config file to my container so that I can fix it every time I need without re-building the image.

I mean I thought the below command would map $(pwd)/config.py from Docker host to /root/app/config.py into the container as a file.

docker run -v $(pwd)/config.py:/root/app/config.py my_docker_image 

However, it always created a directory named config.py, not a file.

while looking for clue, I found the reason(from here)

If you use -v or --volume to bind-mount a file or directory that does not yet exist on the Docker host, -v will create the endpoint for you. It is always created as a directory.

Therefore, it is always created as a directory because my docker host does not have $(pwd)/config.py.

Even if I create config.py in docker host. $(pwd)/config.py just overwirte /root/app/config.py not exporting /root/app/config.py.