Modify volume mounts SELinux labels on the fly based on :Z or :z

This patch is extending the qualifiers on the -v command to allow
an admin to tell the system to relabel, content. There might be a
need for something similar for changing the DAC Permissions.

Signed-off-by: Jessica Frazelle <princess@docker.com>

docs/sources/reference/commandline/cli.md

@@ -2181,6 +2181,19 @@ mount the volumes in read-only or read-write mode, respectively. By default,
 the volumes are mounted in the same mode (read write or read only) as
 the reference container.
 
+Labeling systems like SELinux require proper labels be placed on volume content
+mounted into a container, otherwise the security system might prevent the
+processes running inside the container from using the content. By default,
+volumes are not relabeled.
+
+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. Private volumes can only be used by the
+current container.
+
 The `-a` flag tells `docker run` to bind to the container's `STDIN`, `STDOUT` or
 `STDERR`. This makes it possible to manipulate the output and input as needed.
 
@@ -2222,7 +2235,7 @@ flag:
 	$ docker run --device=/dev/sda:/dev/xvdc --rm -it ubuntu fdisk  /dev/xvdc
 
 	Command (m for help): q
-	$ docker run --device=/dev/sda:/dev/xvdc:r --rm -it ubuntu fdisk  /dev/xvdc
+	$ docker run --device=/dev/sda:/dev/xvdc:ro --rm -it ubuntu fdisk  /dev/xvdc
 	You will not be able to write the partition table.
 
 	Command (m for help): q