for pruning the docker

Dockerfile.prune

 FROM compcert_kvx
 USER root
-RUN apt -y remove opam && apt -y autoremove
+RUN apt-get -y install gcc && apt-get -y remove opam && apt-get -y autoremove
 RUN rm -rf /home/appuser/.opam /home/appuser/CompCert
+USER appuser