extraContainers entries cannot reference other values

`extraContainers` is rendered with plain `toYaml`, so entries are treated as opaque container specs. That makes it impossible to compose an entry's `image` from other values — for example a registry/mirror prefix or a CI-injected tag shared with the chart's first-class image fields. Today the only option is hardcoding a full image string per environment, which drifts from the rest of the chart's image configuration.

Many charts render their extra-container passthroughs through `tpl` so entries may carry template expressions, e.g.:

```yaml
extraContainers:
- name: log-shipper
image: "{{ .Values.someRepository }}:{{ .Values.someTag }}"
```

Proposed fix in #1003 — a one-line change (`tpl (toYaml .Values.extraContainers) .`); plain specs render byte-identical.

Start at the chart template entry point that renders `extraContainers`, using the proposed fix in #1003 as the scope. Verify that template expressions in entries resolve from chart values while plain container specifications remain byte-identical; render the chart with both cases to confirm the result.

Written by the indexing model from the issue text.