As suggested in #1600, there should be a garbage collect option to delete all manifests that are not references by any tags (dangling).

Use case 1:

• push a new tag
• delete an old tag (by deleting a manifest)
• reclaim disk space by running garbage collection

Use case 2:

• push a latest tag (creating "manifest 1")
• push a latest tag (creating "manifest 2")
• reclaim disk space by running garbage collection (delete "manifest 1" automatically)