Clean DNF to make the anaconda-rpm container smaller (#infra)

This is in general good practice.

dockerfile/anaconda-rpm/Dockerfile

@@ -16,6 +16,7 @@ RUN set -e; \
   rpm-build; \
   curl -L https://raw.githubusercontent.com/rhinstaller/anaconda/${git_branch}/anaconda.spec.in | sed 's/@PACKAGE_VERSION@/0/; s/@PACKAGE_RELEASE@/0/; s/%{__python3}/python3/' > /tmp/anaconda.spec; \
   rpmspec -q --buildrequires /tmp/anaconda.spec | xargs -d '\n' dnf install -y; \
+  dnf clean all; \
   mkdir /anaconda
 
 RUN pip install nose