greenlight: map storagedir into container

The storagedir got created, but was never actually used. Map into
greenlight container as was probably the original intention. This
changes ensures that presentations and similar blobs can be
persisted within greenlight.

Fixes: 237740d

templates/greenlight-docker-compose.yml.j2

@@ -16,6 +16,7 @@ services:
 {% endfor %}
     volumes:
       - {{ bbb_greenlight_logdir }}:/usr/src/app/log
+      - {{ bbb_greenlight_storagedir }}:/usr/src/app/storage
 {% if bbb_greenlight_db_adapter == 'sqlite3' %}
       - {{ bbb_greenlight_dbdir }}:/usr/src/app/db/production
 {% endif %}