Revert "chmod container dir to 0770"

This commit broke the testsuite for unprivileged containers as the
container directory is now 0750 with the owner being the container root
and the group being the user's group, meaning that the parent user can
only enter the directory, not create entries in there.

This reverts commit c86da6a3ac517b78e6f710df7efe2f51d153b73c.