Implement cleaning of cached docker images

[craig-willis]

Create celery task (maybe cron) to clear unused and unpinned images. Depends on ability to pin images. Will be a combination of time and available storage (clean oldest images when approaching % of storage limits).

Completion criteria:

• Implement code/celery tasks
• Define test case (manual or integration if possible)
• Create PR
• Deploy to dev

[craig-willis]

This isn't necessary until we allow users to create their own images in v0.7.