When I was trying to run a Jenkins pipeline project, it failed giving this message under the "docker pull node:6-alpine": 

```
<.jenkins/workspace/simple-node-js-react-npm-app@tmp/durable-431710c5/script.sh: line 2: docker: command not found

script returned exit code 127>

```

I have no idea what's going on here, and I couldn't access the directory mentioned in the error. I am pretty new to Jenkins.