Daily garbage collection

[shaunco]

When containers or images are deleted from the registry, the space is never reclaimed from the underlying minio/s3 storage. To resolve this, it would be great to have a daily cron job that calls:

/usr/local/bin/docker-registry garbage-collect /etc/docker-registry.yml --delete-untagged

Daily seems fine, but perhaps the cron frequency expression could be overwritten by an ENV variable. That is, if I want it to only run at 4:05am on Sundays I could set something like this in my docker-compose or helm chart:

GARBAGE_COLLECTION_CRON_EXPRESSION: '5 4 * * sun'

[shaunco]

@dfunckt - happy to provide a PR if the balena team is open to this change