Copy contrib/ to Dockerfile (#9774)

Since Kubespray v2.21.0, the commit a98ab404 removes copying
contrib/ accidentaly. The contrib/ contains useful tools like offline
tools etc. This adds the contrib/ to Dockerfile again.

Dockerfile

@@ -19,6 +19,7 @@ ENV LANG=C.UTF-8
 WORKDIR /kubespray
 COPY *yml /kubespray/
 COPY roles /kubespray/roles
+COPY contrib /kubespray/contrib
 COPY inventory /kubespray/inventory
 COPY library /kubespray/library
 COPY extra_playbooks /kubespray/extra_playbooks