dockerfiles: enable Centos 8 PowerTools

ninja is included in the CentOS PowerTools repository.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/docker/dockerfiles/centos8.docker

@@ -28,5 +28,7 @@ ENV PACKAGES \
     tar \
     zlib-devel
 
-RUN dnf install -y $PACKAGES
+RUN dnf install -y dnf-plugins-core && \
+  dnf config-manager --set-enabled PowerTools && \
+  dnf install -y $PACKAGES
 RUN rpm -q $PACKAGES | sort > /packages.txt