Add detailed documentation about the usage of secrets in codespaces prebuild

[mrclemds]

Code of Conduct

• [X] I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

GitHub Codespaces/Prebuilding your codespaces/Configure prebuilds

What part(s) of the article would you like to see updated?

It would be very helpful to have an example of how to use the secrets from the prebuild action:

• Showing how to retrieve it for the build using a Dockerfile and having it defined as a build-arg (or explain if it is automatically added).
• Showing how to use them on the event commands like onCreateCommand (spawn it using ${SECRET_NAME} as opposed to the action syntax).
• Showing the specificity of the GITHUB_TOKEN (what are its rights? , does it follows the customizations.codespaces.repositories rights ?)
• Showing the specificity of the CODESPACES_PREBUILD_TOKEN (link to Allow external repo access)

Additional information

No response

[welcome[bot]]

Thanks for opening this issue. A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[cmwilson21]

@mrclemds Thanks so much for opening an issue! I'll triage this for the team to take a look :eyes:

[hubwriter]

I will raise this with product management.