Set the permissions for "/" in the ramdisk

Assert that the "/" directory in the ramdisk has the permissions
of 755 and not leave it up to umask (or chance).

Change-Id: I609914f15b8eca6f3cb8e72099c130b88f294224
Closes-Bug: #1770972

roles/modify-image/tasks/manual.yml

@@ -47,6 +47,7 @@
 
   - name: Run script on image
     shell: >
+      chmod 775 {{ mount_tempdir }};
       mount -o bind /dev {{ mount_tempdir }}/dev/;
       mv {{ mount_tempdir }}/etc/resolv.conf{,_};
       echo -e "nameserver 8.8.8.8\nnameserver 8.8.4.4" > {{ mount_tempdir }}/etc/resolv.conf;