Fix setup and CSS for EPUB cover positioning

Make sure the cover is shown in full height.

Signed-off-by: David Runge <dave@sleepmap.de>

book/source/_static/epub/css/custom.css

@@ -66,6 +66,9 @@ Cover
 */
 
 div.cover-img {
-  margin: auto;
-  width: 50%;
+  text-align: center;
 }
+
+div.cover-img span img {
+  height: 100%;
+}

book/source/_templates/cover.j2

@@ -15,6 +15,8 @@ SPDX-License-Identifier: CC-BY-SA-4.0
 
 {% block content %}
   <div class="cover-img">
-    <img src="{{ image }}" alt="Cover image" />
+    <span>
+      <img src="{{ image }}" alt="Cover" />
+    </span>
   </div>
 {% endblock %}