Move into nested docroot

web/core/modules/system/templates/install-page.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation to display a Drupal installation page.
+ *
+ * All available variables are mirrored in page.html.twig.
+ * Some may be blank but they are provided for consistency.
+ *
+ * @see template_preprocess_install_page()
+ *
+ * @ingroup themeable
+ */
+#}
+  <div class="layout-container">
+
+    <header role="banner">
+      {% if site_name or site_slogan %}
+        <div class="name-and-slogan">
+          {% if site_name %}
+            <h1>{{ site_name }}</h1>
+          {% endif %}
+          {% if site_slogan %}
+            <div class="site-slogan">{{ site_slogan }}</div>
+          {% endif %}
+        </div>{# /.name-and-slogan #}
+      {% endif %}
+    </header>
+
+    <main role="main">
+      {% if title %}
+        <h2>{{ title }}</h2>
+      {% endif %}
+      {{ page.highlighted }}
+      {{ page.content }}
+    </main>
+
+    {% if page.sidebar_first %}
+      <aside class="layout-sidebar-first" role="complementary">
+        {{ page.sidebar_first }}
+      </aside>{# /.layout-sidebar-first #}
+    {% endif %}
+
+    {% if page.sidebar_second %}
+      <aside class="layout-sidebar-second" role="complementary">
+        {{ page.sidebar_second }}
+      </aside>{# /.layout-sidebar-second #}
+    {% endif %}
+
+    {% if page.footer %}
+      <footer role="contentinfo">
+        {{ page.footer }}
+      </footer>
+    {% endif %}
+
+  </div>{# /.layout-container #}