Restore rm /var/lib/apt/lists in docker-gitlabci/Dockerfile

It was removed as part of commit 7c8d5b9 (Upgrade gitlab image to
24.04, 2024-04-28), but it should be kept to reduce the size of the
image (if the lists aren't removed in the same RUN directive as
`apt-get update` then they will be stored in the layer and make the
image larger, even if a later RUN directive removes the lists).

docker-gitlabci/Dockerfile

@@ -23,7 +23,8 @@ RUN apt-get update && apt-get install -y \
   # Code coverage for unit test \
   php-pear php-dev \
   # Needed NPM packages \
-  && npm install -g pa11y puppeteer
+  && npm install -g pa11y puppeteer \
+  && rm -rf /var/lib/apt/lists/*
 
 # Install needed global PHP modules
 RUN composer -n require justinrainbow/json-schema