what is 'z' flag in docker container's volumes-from option?

Question

While going through the docker docs, I came across volumes-from (https://docs.docker.com/engine/reference/commandline/run/) option for docker run command. I didn't understand the differences between ro, rw, and z option provided as-
$ docker run --volumes-from ba8c0c54f0f2:ro -i -t ubuntu pwd
In the above command the ro option is replaced with z. I will be thankful if anyone explores on differences of using these options.

Answer 1

Two suffixes :z or :Z can be added to the volume mount. These suffixes tell Docker to relabel file objects on the shared volumes. The 'z' option tells Docker that the volume content will be shared between containers. Docker will label the content with a shared content label. Shared volumes labels allow all containers to read/write content. The 'Z' option tells Docker to label the content with a private unshared label.

https://github.com/rhatdan/docker/blob/e6473011583967df4aa5a62f173fb421cae2bb1e/docs/sources/reference/commandline/cli.md

If you use selinux you can add the z or Z options to modify the selinux label of the host file or directory being mounted into the container. This affects the file or directory on the host machine itself and can have consequences outside of the scope of Docker.

The z option indicates that the bind mount content is shared among multiple containers. The Z option indicates that the bind mount content is private and unshared. Use extreme caution with these options. Bind-mounting a system directory such as /home or /usr with the Z option renders your host machine inoperable and you may need to relabel the host machine files by hand.

$ docker run -d -it --name devtest -v "$(pwd)"/target:/app:z nginx:latest

https://docs.docker.com/storage/bind-mounts/#configure-bind-propagation