Change CMD to -v

Two weeks ago, there was a PR which made Packer's Docker builder copy
CMD and ENTRYPOINT from the base image unless overriden in the packer
build file.  As a result, the RACK Box image now has a non-null CMD
"/bin/bash" coming from the base Ubuntu image, which prevents the RACK
Box from starting when run in Docker.  Make the packer build file
override CMD to "-v" which will allow the RACK Box to start up.

rack-box/rack-box-docker.json

@@ -13,6 +13,7 @@
       "type": "docker",
 
       "changes": [
+        "CMD [ \"-v\" ]",
         "ENTRYPOINT [ \"/usr/bin/python3\", \"/usr/bin/systemctl\" ]"
       ],
       "commit": true,