fix(ci): require docker-capable Jenkins agent with preflight checks

Jenkinsfile

@@ -1,5 +1,7 @@
 pipeline {
-  agent any
+  // Build + deploy stages require Docker CLI/daemon access on the Jenkins node.
+  // Ensure this label maps to an agent with Docker installed and usable.
+  agent { label 'docker' }
 
   environment {
     REGISTRY_IMAGE = "plesk-agency-portal"
@@ -8,6 +10,25 @@ pipeline {
   }
 
   stages {
+    stage('Preflight') {
+      steps {
+        sh '''
+          set -euo pipefail
+
+          if ! command -v docker >/dev/null 2>&1; then
+            echo "Docker CLI is not available on this Jenkins agent."
+            echo "Run this pipeline on a Docker-capable node (label: docker) or install Docker on the current agent."
+            exit 1
+          fi
+
+          if ! docker version >/dev/null 2>&1; then
+            echo "Docker is installed but not usable by Jenkins (daemon/socket/permissions issue)."
+            exit 1
+          fi
+        '''
+      }
+    }
+
     stage('Checkout') {
       steps {
         checkout scm