Add docker availability check to all scripts that use docker commands

22 scripts used docker without verifying it's present. Consistent with the
pattern already established in Docker_Essentials scripts. Added after acquire_lock
and before detect_hosts in each script's setup section.

Rsync/rsync.sh

@@ -165,6 +165,11 @@ fi
 
 acquire_lock
 
+if ! command -v docker &>/dev/null; then
+    error "Docker command not found"
+    exit 1
+fi
+
 validate_unraid_cmd \
     "/usr/local/emhttp/plugins/dynamix/scripts/notify" \
     "" "" \