Routine Garbage Collection of Build Images

[BenGalewsky]

Problem

Docker images are built by the container service and are kept in the local docker image registry forever.

Approach

Add a cron job to the FastAPI server to occasionally issue a docker system prune --all command to clear old layers out. This will make the next build slow since we lose the expensive repo2docker base image.

Make the timing of the purge job configurable through the FastAPI config file.