feat(jeeves): move container state to dedicated ZFS dataset

- add a performance-tuned media/containers dataset
- store Docker and NixOS container state on the dataset
- prepare storage directories after ZFS mounts
- bind NixOS container state to its required path
- configure snapshot retention for container storage

systems/jeeves/snapshot_config.toml

@@ -60,6 +60,12 @@ daily = 30
 monthly = 12
 
 # media
+["media/secure/containers"]
+15_min = 2
+hourly = 0
+daily = 0
+monthly = 0
+
 ["media/temp"]
 15_min = 2
 hourly = 0