Take Docker image content digest into cache key

Created by: mrnugget

This fixes #112 (closed) by fixing a regression I introduced when taking over the src actions exec functionality.

I lower-cased imageContentDigest so that users cannot set it. That had the side effect of src itself not writing the imageContentDigest to the cache key (because json needs the fields to be exported in order to marshal/unmarshal them)

The fix here is to upper-case the field and add a separate validation step to make sure that users don't set the cache field.