Use mount.Target to specify subdirectory of rootfs mount

- Add Target to mount.Mount.
- Add UnmountMounts to unmount a list of mounts in reverse order.
- Add UnmountRecursive to unmount deepest mount first for a given target, using
moby/sys/mountinfo.

Signed-off-by: Edgar Lee <edgarhinshunlee@gmail.com>

docs/snapshotters/README.md

@@ -24,3 +24,37 @@ Filesystem-specific:
 - `nydus`: [Nydus Snapshotter](https://github.com/containerd/nydus-snapshotter)
 - `overlaybd`: [OverlayBD Snapshotter](https://github.com/containerd/accelerated-container-image)
 - `stargz`: [Stargz Snapshotter](https://github.com/containerd/stargz-snapshotter)
+
+## Mount target
+
+Mounts can optionally specify a target to describe submounts in the container's
+rootfs. For example, if the snapshotter wishes to bind mount to a subdirectory
+ontop of an overlayfs mount, they can return the following mounts:
+
+```json
+[
+    {
+        "type": "overlay",
+        "source": "overlay",
+        "options": [
+            "workdir=...",
+            "upperdir=...",
+            "lowerdir=..."
+        ]
+    },
+    {
+        "type": "bind",
+        "source": "/path/on/host",
+        "target": "/path/inside/container",
+        "options": [
+            "ro",
+            "rbind"
+        ]
+    }
+]
+```
+
+However, the mountpoint `/path/inside/container` needs to exist for the bind
+mount, so one of the previous mounts must be responsible for providing that
+directory in the rootfs. In this case, one of the lower dirs of the overlay has
+that directory to enable the bind mount.