Page loading can be slow when docker is not present

And when "latex-pdf-docker” is installed and the LaTeX exporter has not been configured to not use the "docker" method (see https://extensions.xwiki.org/xwiki/bin/view/Extension/LaTeX/Exporter/#HPrerequisites26InstallationInstructions).

Since the UIExtension is called for every page load, the check if docker is up can take too long when it's not available.