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

Answer 1

As of docker-compose file version 3.2, you can specify a volume mount of type "bind" (instead of the default type "volume") that allows you to mount a single file into the container. Search for "bind mount" in the docker-compose volume docs: https://docs.docker.com/compose/compose-file/#volumes

In my case, I was trying to mount a single ".secrets" file into my application that contained secrets for local development and testing only. In production, my application fetches these secrets from AWS instead.

If I mounted this file as a volume using the shorthand syntax:

volumes:
 - ./.secrets:/data/app/.secrets

Docker would create a ".secrets" directory inside the container instead of mapping to the file outside of the container. My code would then raise an error like "IsADirectoryError: [Errno 21] Is a directory: '.secrets'".

I fixed this by using the long-hand syntax instead, specifying my secrets file using a read-only "bind" volume mount:

volumes:
 - type: bind
   source: ./.secrets
   target: /data/app/.secrets
   read_only: true

Now Docker correctly mounts my .secrets file into the container, creating a file inside the container instead of a directory.