On container rm, don't remove named mountpoints

This makes it so when calling `docker run --rm`, or `docker rm -v`, only
volumes specified without a name, e.g. `docker run -v /foo` instead of
`docker run -v awesome:/foo` are removed.

Note that all volumes are named, some are named by the user, some get a
generated name. This is specifically about how the volume was specified
on `run`, assuming that if the user specified it with a name they expect
it to persist after the container is cleaned up.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

man/docker-rm.1.md

@@ -48,6 +48,22 @@ command. The use that name as follows:
 
     docker rm hopeful_morse
 
+## Removing a container and all associated volumes
+
+  $ docker rm -v redis
+  redis
+
+This command will remove the container and any volumes associated with it.
+Note that if a volume was specified with a name, it will not be removed.
+
+  $ docker create -v awesome:/foo -v /bar --name hello redis
+  hello
+  $ docker rm -v hello
+
+In this example, the volume for `/foo` will remain in tact, but the volume for
+`/bar` will be removed. The same behavior holds for volumes inherited with
+`--volumes-from`.
+
 # HISTORY
 April 2014, Originally compiled by William Henry (whenry at redhat dot com)
 based on docker.com source material and internal work.