Update entrypoint in dockerfile for vmpooler gem

This commit updates dockerfile entrypoint to remove the explicit
vmpooler executable path. Additionally, CMD is added to run a default
command when runtime parameters are not passed in by the user. Without
this change the vmpooler executable path is hardcoded into a directory
that will not exist.

docker/Dockerfile

@@ -19,3 +19,5 @@ RUN gem install vmpooler && \
       chmod +x /usr/local/bin/docker-entrypoint.sh
 
 ENTRYPOINT ["docker-entrypoint.sh"]
+
+CMD ["vmpooler"]