$build = [
      '#cache' => [
        'tags'=>['node'], // Invalidate cache whenever a new node is created/modified/deleted
        'contexts'=>['url'] // Invalidate cache per url
      ]
    ];

global $selected_city;
'#cache' => [
    'keys'=>['home_slider_'.$selected_city],
    'tags'=>['node'],
    'contexts'=>['url']
  ]