Added cow option to bind

The option cow can now be set for files which are then
mounted during exec, but copied into the image and removed
if not modified.

Signed-off-by: Christian Goll <cgoll@suse.com>

userdocs/contents/containers.rst

@@ -213,6 +213,21 @@ when using the exec command. This works as follows:
    location, as it is almost always present and empty in every Linux
    distribution (as prescribed by the LSB file hierarchy standard).
 
+Files which should always be present in a container image like ``resolv.conf``
+can be specified in ``warewulf.conf`` with following container_exit
+
+.. code-block:: yaml
+   container mounts:
+   - source: /etc/resolv.conf
+     dest: /etc/resolv.conf
+     readonly: true
+
+.. note::
+   Instead of the ``readonly`` setting you can set ``cow``, which 
+   has the effect, that the source file is copied to the container
+   and removed if it was modified. This useful for file used for
+   registrations.
+
 When the command completes, if anything within the container changed,
 the container will be rebuilt into a bootable static object
 automatically.