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.

Fallback/fallback.sh

@@ -263,6 +263,11 @@ fi
 
 acquire_lock   # strict single instance — notifies on collision
 
+if ! command -v docker &>/dev/null; then
+    error "Docker command not found"
+    exit 1
+fi
+
 detect_hosts
 resolve_remote_ip