Install php-compatibility in our own container to handle the dependencies

We used another container for simplicity of maintainance, as we need to wait for version 10 and we've been for some months now it's easier to put the maintainance in our own container.

Author: vmcj

docker-gitlabci/Dockerfile

@@ -37,7 +37,7 @@ RUN su domjudge -c "cd /home/domjudge && \
                     npm cache clean --force"
 
 # Install needed global PHP modules
-RUN composer -n require justinrainbow/json-schema
+RUN composer -n require justinrainbow/json-schema phpcompatibility/php-compatibility:dev-develop
 
 # Install other PHP versions
 RUN add-apt-repository ppa:ondrej/php -y && apt-get update && \