Dockerfile: Specify a CMD

... as mandated by the Fedora Container Guidelines:
https://fedoraproject.org/wiki/Container:Guidelines#CMD_.2F_ENTRYPOINT

The same entrypoint is also specified in 'create' because since
podman-0.9.1 'podman create ...' doesn't work without a command. See
commit fd697274a3.

Dockerfile

@@ -17,3 +17,5 @@ RUN packages=; while read -r package; do packages="$packages $package"; done \
         <extra-packages; \
     dnf -y install $packages
 RUN rm /extra-packages
+
+CMD /bin/sh