Refactor: harmonize tools/addon and move boilerplate into core.func (#16202)

misc/tools.func

@@ -4628,6 +4628,53 @@ _docker_is_noninteractive() {
   [[ "${DOCKER_NONINTERACTIVE:-}" == "1" || "${DOCKER_NONINTERACTIVE:-}" == "true" || "${DOCKER_NONINTERACTIVE:-}" == "TRUE" ]] || [[ ! -t 0 ]]
 }
 
+# ------------------------------------------------------------------------------
+# ensure_docker()
+#
+# - Docker precondition for addon scripts that ship a compose stack
+# - Reuses an existing installation, otherwise offers to install Docker
+#   (Debian/Ubuntu via setup_docker, Alpine via apk)
+# - Aborts when the user declines or Compose is unavailable afterwards
+# ------------------------------------------------------------------------------
+ensure_docker() {
+  if command -v docker &>/dev/null; then
+    msg_ok "Docker $(docker --version | cut -d' ' -f3 | tr -d ',') is available"
+    if docker compose version &>/dev/null; then
+      msg_ok "Docker Compose is available"
+      return 0
+    fi
+    msg_error "Docker Compose plugin is not available. Please install it."
+    exit 237
+  fi
+
+  msg_warn "Docker is not installed."
+  echo -n "${TAB:-  }Install Docker now? (y/N): "
+  local install_docker_prompt
+  read -r install_docker_prompt
+  if [[ ! "${install_docker_prompt,,}" =~ ^(y|yes)$ ]]; then
+    msg_error "Docker is required for ${APP:-this script}. Exiting."
+    exit 254
+  fi
+
+  if is_alpine; then
+    msg_info "Installing Docker"
+    $STD apk add --no-cache docker docker-cli-compose
+    $STD rc-update add docker default
+    $STD rc-service docker start
+    msg_ok "Installed Docker"
+  else
+    setup_docker || {
+      msg_error "Docker installation failed."
+      exit 237
+    }
+  fi
+
+  if ! command -v docker &>/dev/null || ! docker compose version &>/dev/null; then
+    msg_error "Docker or the Compose plugin is still unavailable after installation."
+    exit 237
+  fi
+}
+
 setup_docker() {
   local docker_installed=false
   local USE_DOCKER_REPO="${USE_DOCKER_REPO:-true}"