Support multiple serverless container redeploys

[jorenvandeweyer]

Summary

• accept comma-separated Scaleway Serverless Container IDs
• redeploy containers once after all image variants are built and pushed
• retain SERVERLESS_CONTAINER_ID as a backward-compatible fallback
• document SERVERLESS_CONTAINER_IDS configuration

Validation

• parsed the changed action and workflow as YAML
• exercised the redeploy loop with single and multiple container IDs
• ran git diff --check

[djbe]

I think we can safely make the old container-id "obsolete", and just start using container-ids generally.

If we want to play it "safer", we can add a small check before rolling out serverless, where we check for the presence of the old var, and throw an error. Or even make it part of the validate inputs step (possibly).