Exec image wiring

[jmcudd]

What does this change

Adds an additional example of how to wire bundled images with exec mixin.

What issue does it fix

It was not obvious how to wire images into a exec mixin and the only example in the documentation was for helm.

Notes for the reviewer

Put any questions or notes for the reviewer here.
In the process of learning how to wire bundled images I found a bug (#3237). There is a workaround to this bug which I didn't include in the documentation, but it might be useful to include it. Here is the workaround.

envs:
  IMAGE_whalesayd: <bundle repository>/my-bundle@${ bundle.images.whalesayd.digest }

This references the whalesayd image that is actually bundled in my-bundle. In all my testing, bundle.images.whalesayd.repository is actaully just referencing the upstream image, which is potentially not accessible in an airgapped environment.

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
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[kichristensen]

@jmcudd Thanks for adding this to the documentation