Correct the stale root .dockerignore comment about retriever's build context

[ckrough]

Found while fixing the container-build (retriever) failure on PR #321.

The root .dockerignore header says:

Other services (retriever, stacker) build from their own service-root contexts and read their own .dockerignore, so this file does not affect them.

That is wrong for retriever. services/retriever/wrangler.jsonc sets image_build_context: "../..", so retriever builds from the repo root and does read the root .dockerignore, exactly as petdata does. Both services that consume a packages/* path source build from the repo root, because a service-root context puts ../../packages/* outside the context and breaks uv sync --frozen.

No behavior is wrong today: the root .dockerignore excludes nothing under packages/ or services/, and its own next paragraph already warns not to. The defect is that the comment tells a reader the file does not affect retriever, so an exclusion added on that basis would break the retriever image build, and the failure surfaces as a uv sync resolution error rather than anything pointing at .dockerignore.

• Correct the header to name retriever and petdata as repo-root-context builds that read this file, and stacker as the service-root exception.
• Keep the existing "do NOT exclude packages/ or services/" warning, and state that it applies to both services.

Acceptance Criteria

• The root .dockerignore comment matches the image_build_context values actually set in each service's wrangler.jsonc.
• No change to the ignore patterns themselves.