Make it so the Docker image can put the `audio_blobs/` directory on an external volume

[rozbb]

Currently, audio_blobs/ lives inside the Docker image. That means 1) it is limited by whatever space constraints the docker runner is using, and 2) whenever the image is torn down, so is all the data.

It would be preferable to have the Dockerfile (or docker-compose file?) specify an external location to persistently keep the audio_blobs/ data.