Rework changes to libpod.conf

In order to be sure, that we stay up-to-date with the defaults and not
missing anything, it's better to use sed instead of an own config file.

This patch should works the previous file integration with an sed based
approach.

Dockerfile

 FROM fedora:30
 
-COPY resources/storage.conf resources/libpod.conf /etc/containers/
+COPY resources/storage.conf /etc/containers/
 
-RUN dnf -y upgrade && dnf -y install podman buildah findutils && dnf clean all
+RUN true\
+   && dnf -y upgrade \
+   && dnf -y install podman buildah findutils \
+   && dnf clean all \
+   && sed -e 's/cgroup_manager =.*/cgroup_manager = "cgroupfs"/' /usr/share/containers/libpod.conf > /etc/containers/libpod.conf \
+   && true
 
 CMD ["/bin/bash"]

resources/libpod.conf

-cgroup_manager = "cgroupfs"