docs(components): explain a container's filesystem

This explains that a container's filesystem is ephemeral, and
that any changes made to disk will be wiped when the container
is either stopped or restarted.

Author: Matthew Fisher

docs/components/container.rst

@@ -7,7 +7,16 @@
 Container
 =========
 Deis containers are instances of `Docker`_ containers used to run :ref:`Applications <application>`.
-Containers perform the actual work of an :ref:`application` by servicing requests or running 
+Containers perform the actual work of an :ref:`application` by servicing requests or running
 background tasks as part of the :ref:`Formation's <formation>` runtime :ref:`Layer`.
 
-.. _`Docker`: http://docker.io/
+Ephemeral Filesystem
+--------------------
+
+Each container gets its own ephemeral filesystem, with a fresh copy of the most recently
+deployed code. During the container’s lifetime, its running processes can use the
+filesystem as a temporary scratchpad, but no files that are written are visible to
+processes in any other container. Any files written to the ephemeral filesystem will be
+discarded the moment the container is either stopped or restarted.
+
+.. _`Docker`: http://docker.io/