Fix: properly link external images

This works by prepending the site's `base_url` to internal
image paths. Image URLs are deemed to be external if they
begin with `http`.

This fix was added to a PR for the Terminimal theme.

PR: pawroman/zola-theme-terminimal#37

templates/shortcodes/image.html

@@ -1,3 +1,8 @@
 {% if src %}
-  <img src="{{ config.base_url ~ src | safe }}"{% if alt %} alt="{{ alt }}"{% endif %} class="{% if position %}{{ position }}{% else -%} center {%- endif %}" {%- if style %} style="{{ style | safe }}" {%- endif %} />
+  {# If the image's URL is internal to the site... #}
+  {% if src is not starting_with("http") %}
+    {# ... then prepend the site's base URL to the image's URL. #}
+    {% set src = config.base_url ~ src %}
+  {% endif %}
+  <img src="{{ src | safe }}"{% if alt %} alt="{{ alt }}"{% endif %} class="{% if position %}{{ position }}{% else -%} center {%- endif %}" {%- if style %} style="{{ style | safe }}" {%- endif %} />
 {% endif %}