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>

mount/mount_linux_test.go

@@ -172,3 +172,67 @@ func TestMountAt(t *testing.T) {
 		t.Fatalf("unexpected working directory: %s", newWD)
 	}
 }
+
+func TestUnmountMounts(t *testing.T) {
+	testutil.RequiresRoot(t)
+
+	target, mounts := setupMounts(t)
+	if err := UnmountMounts(mounts, target, 0); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestUnmountRecursive(t *testing.T) {
+	testutil.RequiresRoot(t)
+
+	target, _ := setupMounts(t)
+	if err := UnmountRecursive(target, 0); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func setupMounts(t *testing.T) (target string, mounts []Mount) {
+	dir1 := t.TempDir()
+	dir2 := t.TempDir()
+
+	if err := os.Mkdir(filepath.Join(dir1, "foo"), 0755); err != nil {
+		t.Fatal(err)
+	}
+	mounts = append(mounts, Mount{
+		Type:   "bind",
+		Source: dir1,
+		Options: []string{
+			"ro",
+			"rbind",
+		},
+	})
+
+	if err := os.WriteFile(filepath.Join(dir2, "bar"), []byte("bar"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	mounts = append(mounts, Mount{
+		Type:   "bind",
+		Source: dir2,
+		Target: "foo",
+		Options: []string{
+			"ro",
+			"rbind",
+		},
+	})
+
+	target = t.TempDir()
+	if err := All(mounts, target); err != nil {
+		t.Fatal(err)
+	}
+
+	b, err := os.ReadFile(filepath.Join(target, "foo/bar"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if string(b) != "bar" {
+		t.Fatalf("unexpected file content: %s", b)
+	}
+
+	return target, mounts
+}