Docker, mount volumes as readonly

Question

I am working with Docker, and I want to mount a dynamic folder that changes a lot (so I would not have to make a Docker image for each execution, which would be too costly), but I want that folder to be read-only. Changing the folder owner to someone else works. However, chown requires root access, which I would prefer not to expose to an application.

When I use -v flag to mount, it gives whatever the username I give, I created a non-root user inside the docker image, however, all the files in the volume with the owner as the user that ran docker, changes into the user I give from the command line, so I cannot make read-only files and folders. How can I prevent this?

I also added mustafa ALL=(docker) NOPASSWD: /usr/bin/docker, so I could change to another user via terminal, but still, the files have permissions for my user.

Answer 1

You can specify that a volume should be read-only by appending :ro to the -v switch:

docker run -v volume-name:/path/in/container:ro my/image

Note that the folder is then read-only in the container and read-write on the host.

2018 Edit

According to the Use volumes documentation, there is now another way to mount volumes by using the --mount switch. Here is how to utilize that with read-only:

$ docker run --mount source=volume-name,destination=/path/in/container,readonly my/image

docker-compose

Here is an example on how to specify read-only containers in docker-compose:

version: "3"
services:
  redis:
    image: redis:alpine
    read_only: true