mosquitto does not create pid file

I'm trying to test the new file-based runtime config using a Docker image. On the whole the config file is read and the config options are loaded.

The issue at the moment is that the sample conf file (findserver.conf) specifies that the mosquitto.pid should be read from /var/run/mosquitto.pid which seems logical and according to the mosquitto documentation.

However /var/run/mosquitto.pid is never created in the Docker image and causes findserver to terminate. In fact I can't seem to find it anywhere within the image.

At a guess I'd say there's an issue with mosquitto within the image or maybe it doesn't have correct permissions to create the pid file. Any ideas what's going on?