How can I change permission of mounted volumes in docker-compose.yml from the docker-compose.yml?

When bind-mounting a directory from the host in a container, files and directories maintain the permissions they have on the host. This is by design: when using a bind-mount, you're giving the container access to existing files from the host, and Docker won't make modifications to those files; doing so would be very dangerous (for example, bind-mounting your home-directory would change file permissions of your host's home directory, possibly leading to your machine no longer being usable).

To change permissions of those files, change their permissions on the host.

You can find more information on this in another answer I posted on StackOverflow: https://stackoverflow.com/a/29251160/1811501

you can add the permissions after an extra column like:

volumes:
        - ./:/var/www:ro //read only