Document ignition-validate usage (was: Document better how to use the Docker image of ct to generate the Ignition config)

[pothos]

Current situation

For new users it is mostly unclear in the docs which format is used: either the ready-to-use Ignition config as JSON or, more often, the Container Linux Config as YAML which needs to be converted to Ignition first.
The docs about the Container Linux Config mention how to use the transpiler but not that there is a Docker image for easy usage: https://docs.flatcar-linux.org/os/provisioning/
The docs about the transpiler don't mention the Docker image either: https://docs.flatcar-linux.org/container-linux-config-transpiler/doc/overview/

Impact
When a config is presented in the docs, it is not clear sometimes that it needs transpilation to be used.
How transpilation is used in an easy way with Docker is not explained.

Ideal future situation
Show two tabs, one with CL Config and one with Ignition (see for example https://coreos.com/os/docs/latest/adding-users.html).

Include how to use the transpiler with Docker in https://docs.flatcar-linux.org/container-linux-config-transpiler/doc/getting-started/

cat cl-config.yaml | docker run --rm -i quay.io/coreos/ct:latest-dev > ignition.json

Consider to host the Docker image on our quay.io account.
https://quay.io/repository/coreos/ct

Implementation options
–

Additional information
There is also a validator image that seems to be used on https://coreos.com/validate/:
https://quay.io/repository/coreos/ignition-validate

Consider to document it in the Flatcar docs or even publish a similar web page, too.

[vbatts]

Yes, please. This is a learning curve.

[pothos]

ct Docker image in docs is done

• The validation with ignition-validate is not yet mentioned in the docs.

Jeremi made a PoC for a transpilation website.