How to give non-root user in Docker container access to a volume mounted on the host

I am running my application in a Docker container as a non-root user. I did this since it is one of the best practices. However, while running the container I mount a host volume to it -v /some/folder:/some/folder . I am doing this because my application running inside the docker container needs to write files to the mounted host folder. But since I am running my application as a non-root user, it doesn't have permission to write to that folder

Question

Is it possible to give a nonroot user in a docker container access to the hosted volume?

If not, is my only option to run the process in docker container as root?

There's no magic solution here: permissions inside docker are managed the same as permissions without docker. You need to run the appropriate chown and chmod commands to change the permissions of the directory.

One solution is to have your container run as root and use an ENTRYPOINT script to make the appropriate permission changes, and then your CMD as an unprivileged user. For example, put the following in entrypoint.sh:

#!/bin/sh

chown -R appuser:appgroup /path/to/volume
exec runuser -u appuser "$@"

This assumes you have the runuser command available. You can accomplish pretty much the same thing using sudo instead.

Use the above script by including an ENTRYPOINT directive in your Dockerfile:

FROM baseimage

COPY entrypoint.sh /entrypoint.sh
ENTRYPOINT ["/bin/sh", "entrypoint.sh"]
CMD ["/usr/bin/myapp"]

This will start the container with:

/bin/sh entrypoint.sh /usr/bin/myapp

The entrypoint script will make the required permissions changes, then run /usr/bin/myapp as appuser.

There will throw error if host env don't have appuser or appgroup, so better to use a User ID instead of user name:

inside your container, run

appuser$ id

This will show:

> uid=1000(appuser) gid=1000(appuser) groups=1000(appuser)

From host env, run:

mkdir -p /some/folder
chown -R 1000:1000 /some/folder
docker run -v /some/folder:/some/folder [your_container]

inside your container, check

ls -lh

to see the user and group name, if it's not root, then it's should worked.

In the specific situation of using an image built from a custom Dockerfile, you can do the following (using example commands for a debian image):

    FROM baseimage
    ...
    RUN useradd --create-home appuser
    USER appuser
    RUN mkdir /home/appuser/my_volume
    ...

Then mount the volume using

-v /some/folder:/home/appuser/my_volume

Now appuser has write permissions to the volume as it's in their home directory. If the volume has to be mounted outside of their home directory, you can create it and assign appuser write permissions as an extra step within the Dockerfile.