plugins: container-rootfs-relative paths

Legacy plugins expect host-relative paths (such as for Volume.Mount).
However, a containerized plugin cannot respond with a host-relative
path. Therefore, this commit modifies new volume plugins' paths in Mount
and List to prepend the container's rootfs path.

This introduces a new PropagatedMount field in the Plugin Config.
When it is set for volume plugins, RootfsPropagation is set to rshared
and the path specified by PropagatedMount is bind-mounted with rshared
prior to launching the container. This is so that the daemon code can
access the paths returned by the plugin from the host mount namespace.

Signed-off-by: Tibor Vass <tibor@docker.com>

docs/extend/config.md

@@ -111,6 +111,10 @@ Config provides the base accessible fields for working with V0 plugin format
 
 	  options of the mount.
 
+- **`propagatedMount`** *string*
+
+   path to be mounted as rshared, so that mounts under that path are visible to docker. This is useful for volume plugins.
+
 - **`env`** *PluginEnv array*
 
    env of the plugin, struct consisting of the following fields

docs/extend/plugins_volume.md

@@ -22,6 +22,10 @@ beyond the lifetime of a single Engine host. See the
 
 ## Changelog
 
+### 1.13.0
+
+- If used as part of the v2 plugin architecture, mountpoints that are part of paths returned by plugin have to be mounted under the directory specified by PropagatedMount in the plugin configuration [#26398](https://github.com/docker/docker/pull/26398)
+
 ### 1.12.0
 
 - Add `Status` field to `VolumeDriver.Get` response ([#21006](https://github.com/docker/docker/pull/21006#))