feat: Support placeholder substitution in container image

[joeljeske]

Description

Placeholders already resolve in containers.*.variables, containers.*.files,
containers.*.volumes.source and resources.*.params, but
containers.*.image was passed through verbatim. This resolves the image with
the same substituter already used for variables, so it sees exactly the same
refs:

What does this PR do?

It makes the image field composable from resource outputs and workload
metadata, instead of requiring the caller to rewrite the score file or pass
--image. --image takes a single value per invocation, so it can't serve
multiple workloads generated together, and it only applies to image: ".".

The omission looks incidental rather than deliberate: the substitution
machinery is generic, resources are fully provisioned before conversion so
outputs are already available, and no test asserted that a placeholder in
image stays literal.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• New chore (expected functionality to be implemented)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've signed off with an email address that matches the commit author.

Related: score-spec/score-compose#538

[mathieu-benoit]

Thanks for your contribution, @joeljeske!

The DCO is failing, to fix it you can run this in your local branch:

git rebase HEAD~1 --signoff
git push --force-with-lease origin feat/image-ref