docs(contribute): clarify service compose templates

[vaguul]

Changes

• Add a short service-template Docker Compose checklist to the contribution guide.
• Clarify version-pinned images, Coolify-generated variables/storage, static container names, restart policies, and healthchecks.
• Update the required-env example to use a versioned image tag instead of latest.

Issue

• Closes New Content: docker-compose clarification #500

Testing

• git diff --check
• Not run: local docs build/typecheck, because this checkout has no bun command or node_modules installed.