Docker image needs Ruby, too
Docker image needs Ruby, too

fixes:
```
Installing CPI:
  Rendering and uploading Jobs:
    Rendering job templates for installation:
      Rendering templates for job 'google_cpi/774d9cd24e90268d6df3026bc7ce4ab43dbf0984':
        Rendering template src: config/cpi.json.erb, dst: config/cpi.json:
          Rendering template src: /root/.bosh/installations/7e5b0151-3544-4b23-6321-5cf1da75ff25/tmp/bosh-release-job484242362/templates/config/cpi.json.erb, dst: /root/.bosh/installations/7e5b0151-3544-4b23-6321-5cf1da75ff25/tmp/rendered-jobs344476236/config/cpi.json:
            Running ruby to render templates:
              Starting command 'ruby /root/.bosh/installations/7e5b0151-3544-4b23-6321-5cf1da75ff25/tmp/erb-renderer596681531/erb-render.rb /root/.bosh/installations/7e5b0151-3544-4b23-6321-5cf1da75ff25/tmp/erb-renderer596681531/erb-context.json /root/.bosh/installations/7e5b0151-3544-4b23-6321-5cf1da75ff25/tmp/bosh-release-job484242362/templates/config/cpi.json.erb /root/.bosh/installations/7e5b0151-3544-4b23-6321-5cf1da75ff25/tmp/rendered-jobs344476236/config/cpi.json':
                exec: "ruby": executable file not found in $PATH
```

cunnie/deployments,cunnie/deployments