Added saving buildcontainer to cache directory

docker/buildcontainer/build_circle.sh

@@ -8,6 +8,7 @@ if [[ -e ~/docker/centos.tar ]]; then
   docker load -i ~/docker/centos.tar;
 else
   docker build --tag "grafana/buildcontainer" docker/buildcontainer
+  docker save grafana:buildcontainer > ~/docker/centos.tar;
 fi