Make sure $TMP_BUILD_DIR/mnt is owned by root

The path $TMP_BUILD_DIR/mnt becomes the / inside the chroot during
the chroot phases of diskimage-builder. Previously this path was being
created using the account running diskimage-builder. This account may
not be valid inside the chroot. This causes path validation, when running
on a Ubuntu bionic host, to fail.
This patch chown's the $TMP_BUILD_DIR/mnt to root.root to make sure
that / is owned by a valid account inside the chroot.

Change-Id: Ifedc136baa67c7952942aed2c8cb1041902fef91
Closes-Bug: 1811113

diskimage_builder/lib/common-functions

@@ -339,6 +339,10 @@ function arg_to_elements() {
 
 function create_base () {
     mkdir $TMP_BUILD_DIR/mnt
+    # Make sure the / inside the chroot is owned by root
+    # If it is not owned by root, some Ubuntu bionic packages will fail
+    # path validation at install time.
+    sudo chown root.root $TMP_BUILD_DIR/mnt
     export TMP_MOUNT_PATH=$TMP_BUILD_DIR/mnt
     # Copy data in to the root.
     TARGET_ROOT=$TMP_MOUNT_PATH run_d root