Try to make importing SQLite from older Synapse version work

If the SQLite database was from an older version of Synapse, it appears
that Synapse would try to run migrations on it first, before importing.
This was failing, because the file wasn't writable.

Hopefully, this fixes the problem.

roles/matrix-postgres/tasks/import_sqlite_db.yml

@@ -81,6 +81,6 @@
     --entrypoint=python
     -v {{ matrix_synapse_config_dir_path }}:/data
     -v {{ matrix_synapse_config_dir_path }}:/matrix-media-store-parent/media-store
-    -v {{ server_path_homeserver_db }}:/{{ server_path_homeserver_db|basename }}:ro
+    -v {{ server_path_homeserver_db }}:/{{ server_path_homeserver_db|basename }}
     {{ matrix_synapse_docker_image }}
     /usr/local/bin/synapse_port_db --sqlite-database /{{ server_path_homeserver_db|basename }} --postgres-config /data/homeserver.yaml