Add support for mount in runtime config

This could be used in LinuxKit now, as there are some examples, eg
https://github.com/linuxkit/linuxkit/blob/master/blueprints/docker-for-mac/base.yml#L33
which are creating containers to do a mount.

The main reason though is to in future change the ad hoc code that generates
overlay mounts for writeable containers with a runtime config which does
the same thing; this code needs to create both tmpfs and overlay mounts.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

src/moby/schema.go

@@ -239,6 +239,7 @@ var schema = string(`
       "type": "object",
       "additionalProperties": false,
       "properties": {
+        "mounts": {"$ref": "#/definitions/mounts"},
         "mkdir": {"$ref": "#/definitions/strings"},
         "interfaces": {"$ref": "#/definitions/interfaces"},
         "bindNS": {"$ref": "#/definitions/namespaces"}