[docker_test.sh] Manage special condition where folder is present before pulling but not secret files

More seriously, this step was useful once upon a time because it also used to build the base image for each commit. Now that we have pica-ci-base, this step was only useful to determine what folder have been modified and export some environement variable in a cached file. But Gitlab has race conditions with cache and it will randomly fail, annoying the users of the CI. We will execute the script to set these environment variables at each step of the CI, with a really minimal overhead.