[CI] Use lighter Emacs image, remove validation links in HTML

The docker image used is now Emacs on Alpine, which should be lighter
than the previous one.

The validation link is also now disabled in HTML exports.

export.el

@@ -6,7 +6,8 @@
 (package-install 'htmlize)
 (package-install 's)
 (require 's)
-(setq org-confirm-babel-evaluate nil)
+(setq org-confirm-babel-evaluate nil
+      org-html-validation-link nil)
 (let* ((files (mapcar #'expand-file-name
                       (file-expand-wildcards "org/config/*.org"))))
   (mapc (lambda (file)