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

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

docker run --volumes-from a64f10cd5f0e:z -i -t rhel6 bin/bash

I have tested it, i have mounted in one container and from that container to another newly container. IT goes with rw option

From tests here in my machine, -z lets you share content from one container with another. Suppose this image:

FROM alpine
RUN mkdir -p /var/www/html \
    && echo "foo" > /var/www/html/index.html

Let's build it and tag as test-z:

$ docker build . -t test-z

Now create and run test-z container with the name testing-z, mapping the volume test-vol to /var/www/html and adding the z modifier

$ docker run \
    --name testing-z \
    --volume test-vol:/var/www/html:z \
    -d test-z tail -f /dev/null

The contents of /var/www/html from testing-z can be accessed from others containers by using the --volumes-from flag, like below:

$ docker run --rm --volumes-from testing-z -it nginx sh
# cat /var/www/html/index.html
foo

Obs.: I'm running Docker version 19.03.5-ce, build 633a0ea838