try some stuff

Dockerfile

@@ -4,6 +4,7 @@ FROM php:7.4-apache as base
 # Install system requirements
 RUN apt update && apt install -y  --no-install-recommends \
     default-mysql-client default-libmysqlclient-dev libcurl4-openssl-dev zlib1g-dev libpng-dev libonig-dev libzip-dev libicu-dev \
+    cron \
     && rm -rf /var/lib/apt/lists/*
 
 # Install php extensions
@@ -53,6 +54,11 @@ COPY config_local.php /config/config_local.inc.php
 COPY docker-entrypoint.sh /usr/local/bin/
 RUN chmod u+x /usr/local/bin/docker-entrypoint.sh
 
+# Add Cronjob
+COPY crontab /etc/cron.d/crontab
+RUN chmod 0644 /etc/cron.d/crontab
+RUN crontab /etc/cron.d/crontab
+
 # Set start parameters
 ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
 CMD ["apache2-foreground"]

crontab

+* * * * * root php /var/www/studip/cli/cronjob-worker.php >> /dev/null 2>&1

docker-entrypoint.sh

@@ -64,4 +64,7 @@ if [ "${1#-}" != "$1" ]; then
 	set -- apache2-foreground "$@"
 fi
 
+# Start cron
+cron
+
 exec "$@"