repository docker.io/stackstorm/hubot not found: does not exist or no pull access

Hi team.

When I tried to pull st2chatops docker image, I got below error.

```
root# docker pull stackstorm/hubotUsing default tag: latest
Trying to pull repository docker.io/stackstorm/hubot ...
repository docker.io/stackstorm/hubot not found: does not exist or no pull access
```

It seems like my shell cannot find the docker image.