How is the Docker Mount Point decided?

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I pull an image from Docker Hub (say) Ghost CMS and after reading the documentation, I see that the default mount point is /var/lib/ghost/content

Now, when I make my own application with Ghost as the base image, I map some folder (say) CMS-Content and mount it on /var/lib/ghost/content written like this -

volumes:
 - CMS-Content:  /var/lib/ghost/content

The path /var/lib/ghost/content are System Level paths. However, CMS-Content is a folder I created to host my files (persistent data).

Finally, I decide to publish my application as an image in Docker Hub, so what will be the mount point now?

volumes docker-compose docker mount containers
2 Answers

If you want to make a pesistent data for the container :

Using command-line :

docker run -it --name <WHATEVER> -p <LOCAL_PORT>:<CONTAINER_PORT> -v <LOCAL_PATH>:<CONTAINER_PATH> -d <IMAGE>:<TAG>

Using docker-compose.yaml :

version: '2'
  services:
    cms:
       image: <IMAGE>:<TAG>
       ports:
       - <LOCAL_PORT>:<CONTAINER_PORT>
       volumes:
       - <LOCAL_PATH>:<CONTAINER_PATH>

Assume :

  • IMAGE: ghost-cms
  • TAG: latest
  • LOCAL_PORT: 8080
  • CONTAINER_PORT: 8080
  • LOCAL_PATH: /persistent-volume
  • CONTAINER_PATH: /var/lib/ghost/content

Examples :

  1. First create /persistent-volume.
$ mkdir -p /persistent-volume
  1. docker-compose -f docker-compose.yaml up -d
version: '2'
  services:
    cms:
       image: ghost-cms:latest
       ports:
       - 8080:8080
       volumes:
       - /persistent-volume:/var/lib/ghost/content
user_41157520

Each container has its own isolated filesystem. Whoever writes an image's Dockerfile gets to decide what filesystem paths it uses, but since it's isolated from other containers and the host it's very normal to use "system" paths. As another example, the standard Docker Hub database images use /var/lib/mysql and /var/lib/postgresql/data. For a custom application you might choose to use /app/data or even just /data, if that makes sense for you.

If you're creating an image FROM a pre-existing image like this, you'll usually inherit its filesystem layout, so the mount point for your custom image would be the same as in the base image.

Flipping through the Ghost Tutorials, it looks like most things you could want to do either involve using the admin UI or making manual changes in the content directory. The only files that changes are in the CMS-Content named volume in your example (and even if you didn't name a volume, the Docker Hub ghost image specifies an anonymous volume there). That means you can't create a derived image with a standard theme or other similar setup: you can't change the content directory in a derived image, and if you experiment with docker commit (not recommended) the image it produces won't have the content from the volume.

user_100081730
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