docs: Provide Dockerfile example for validating Cloud-Init user data

[akutz]

Getting cloud-init user data correct may prove difficult for users. The following Dockerfile can be used to validate the user data:

FROM ubuntu:latest

RUN apt-get update -y && \
    apt-get install -y cloud-init

ENTRYPOINT [ "cloud-init", "schema", "--config-file", "/cloud-config.yaml" ]

Let's imagine we build the following image from the above Dockerfile:

docker build -t civ .

Now, imagine the following cloud config file exists as "my-config.yaml":

#cloud-config
ssh_pwauth: true
users:
    - default
    - name: vmware
      sudo: ALL=(ALL) NOPASSWD:ALL
      lock_passwd: false
      passwd: '$1$salt$SOC44fVbA/ZxeIwD5yw1u1'
      shell: /bin/bash
write_files:
    - content: |
      "VMSVC Says Hello World"
      path: /helloworld

Let's run the validator on the above file:

$ docker run -it --rm -v $(pwd)/my-config.yaml:/cloud-config.yaml:ro civ
Error:
Cloud config schema errors: format-l13.c7: File /cloud-config.yaml is not valid yaml. while scanning a simple key
  in "<byte string>", line 13, column 7:
          "VMSVC Says Hello World"
          ^
could not find expected ':'
  in "<byte string>", line 14, column 7:
          path: /helloworld
          ^

We should add documentation to this repo's doc area with the above example.

[ChrisSmith2]

Hey!
I'm a computer science student at the University of Texas at Austin taking a Virtualization course and one of our assignments is to contribute to an open source project. Would I be able to work on resolving this issue?

[akutz]

Hi @ChrisSmith2,

How is the ol' 40 acres? I'm the tech lead on the VM Service feature at VMware, of which the VM Operator is a part, and I roamed the halls of the old CS building back when Edsger Dijkstra was still teaching. Are Drs. Novak and Downing still there? If so, be sure to take a LISP course from Professor Novak, you'll seldom have as much fun!

You would be more than welcome to have a go at this; thank you for being interested! If you need any help, please feel free to reach out here and ask!

[MatthewDulworth]

Hey @akutz I am @ChrisSmith2's partner. Novak is indeed still here, I took a LISP class from him two semesters ago!

Is there any info you want use to add to the docs, or just put your example under the Cloud Init section in the Deploying a VM tutorial?