docker-stacks-update: add -s/--stack to target specific stacks

Usage:  docker-stacks-update -s stack_a -s stack_b

Only matching running stacks are processed. Shows a helpful message
with available stacks if no match is found.

docker-stacks-update/docker-stacks-update.sh

@@ -20,6 +20,8 @@ if command -v tput >/dev/null 2>&1 && [ -t 1 ]; then
 fi
 
 # ── State ───────────────────────────────────────────────────────────
+TARGET_STACKS=()     # user-specified stacks (empty = all)
+all_names=()         # pre-filter stack names (for error messages)
 updated_names=()     # stack names that were updated
 updated_detail=()    # parallel: "stack: service1, service2"
 unchanged_names=()   # stack names already up to date
@@ -43,6 +45,7 @@ that have updates.
 
 Options:
   -h, --help              Show this help message
+  -s, --stack <name>      Target a specific stack (repeatable)
 
 No options = update all running stacks.
 EOF
@@ -50,10 +53,19 @@ EOF
 }
 
 # ── Parse arguments ──────────────────────────────────────────────────
-for arg in "$@"; do
-  case "$arg" in
+while [[ $# -gt 0 ]]; do
+  case "$1" in
     -h|--help) usage ;;
-    *) echo -e " ${RED}✗${RESET} Unknown option: ${arg}" >&2; usage ;;
+    -s|--stack)
+      shift
+      [[ $# -eq 0 ]] && { echo -e " ${RED}✗${RESET} Option --stack requires a name." >&2; exit 1; }
+      TARGET_STACKS+=("$1")
+      shift
+      ;;
+    -*)
+      echo -e " ${RED}✗${RESET} Unknown option: $1" >&2; usage ;;
+    *)
+      echo -e " ${RED}✗${RESET} Unexpected argument: $1" >&2; usage ;;
   esac
 done
 
@@ -117,6 +129,31 @@ _spin_stop() {
   echo -ne "\r\033[K"
 }
 
+# ── Filter stacks if --stack was given ───────────────────────────────
+if [[ ${#TARGET_STACKS[@]} -gt 0 ]]; then
+  all_names=("${names[@]}")
+  filtered_names=()
+  filtered_configs=()
+  for i in "${!names[@]}"; do
+    for t in "${TARGET_STACKS[@]}"; do
+      if [[ "${names[$i]}" == "$t" ]]; then
+        filtered_names+=("${names[$i]}")
+        filtered_configs+=("${configs[$i]}")
+        break
+      fi
+    done
+  done
+  names=("${filtered_names[@]}")
+  configs=("${filtered_configs[@]}")
+  if [[ ${#names[@]} -eq 0 ]]; then
+    warn "No running stacks match the requested name(s)."
+    echo "  ${CYAN}→${RESET} Running stacks: ${all_names[*]}"
+    exit 0
+  fi
+  echo "  ${CYAN}→${RESET} Targeting ${BOLD}${#names[@]}${RESET} of ${BOLD}${total}${RESET} running stack(s)"
+  echo ""
+fi
+
 # ── Process each stack ──────────────────────────────────────────────
 for i in "${!names[@]}"; do
   stack="${names[$i]}"