Expose docker tag information using the API

This information is available since the introduction of `versions` file.
Docker info is shown by `pihole -v` and the web interface footer, but
it was never added to the versions API.

Signed-off-by: RD WebDesign <github@rdwebdesign.com.br>

api.php

@@ -71,6 +71,13 @@
     $branches = array('core_branch' => $core_branch,
         'web_branch' => $web_branch,
         'FTL_branch' => $FTL_branch, );
+    if (isset($versions['DOCKER_VERSION'])) {
+        // Docker info is available only inside containers
+        $updates['docker_update'] = $docker_update;
+        $current['docker_current'] = $docker_current;
+        $latest['docker_latest'] = $docker_latest;
+    }
+
     $data = array_merge($data, $updates);
     $data = array_merge($data, $current);
     $data = array_merge($data, $latest);