Docker Volumes are not accessible to non-root container users

[buvoli]

This affects cjr $ command when run with --file-access=volume. Since files in all docker volumes are owned by root (moby/moby#3124) non root container users cannot access them. If an image creates non root users then the cjr $ commands will fail if they requires file manipulation. A possible fix would be to introduce new flag for docker. If selected the chown files to (id -i) after rsync.