fix: yum update during image creation (#11)

Run yum update as show in [the ubi8 docs](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/building_running_and_managing_containers/adding-software-to-a-running-ubi-container_building-running-and-managing-containers#building-an-ubi-based-image_adding-software-to-a-running-ubi-container).

Dockerfile

@@ -5,6 +5,7 @@ LABEL org.opencontainers.image.vendor="Adfinis"
 
 COPY backup.sh /usr/local/bin/backup.sh
 
+RUN microdnf update -y && rm -rf /var/cache/yum
 RUN microdnf install findutils -y && microdnf clean all
 
 CMD ["/usr/local/bin/backup.sh"]