fix(stack/signal-processing): Namespace interpolation

Add extraEnv for providing the templated namespace.
We would use the downward api, but that breaks because the value of extraEnv is also passed to `hub` which expects a dict.

Author: NickLarsenNZ

stacks/signal-processing/jupyterhub.yaml

@@ -58,14 +58,17 @@ options:
       # {% if DEMO is defined %}
       stackable.tech/demo: "{{ DEMO }}"
       # {% endif %}
+    extraEnv:
+      # Needed because the Notebook runs code to interpolate the Postgres DB hostname with a namespace.
+      # We cannot use the downward API because some python code uses it and expects scalar values.
+      # We also MUST use a dict, otherwise hub shows the following error:
+      # traitlets.traitlets.TraitError: The 'environment' trait of a KubeSpawner instance expected a dict, not the list [{'name': 'NAMESPACE', 'value': 'my-namespace'}].
+      # A helper in the chart formats these properly for any container env vars in Pods.
+      NAMESPACE: "{{ NAMESPACE }}"
     initContainers:
       - name: download-notebook
         image: oci.stackable.tech/sdp/tools:1.0.0-stackable0.0.0-dev
         env:
-          - name: NAMESPACE
-            valueFrom:
-              fieldRef:
-                fieldPath: metadata.namespace
         # This file path needs adjustment for versioned demos on release branch
         command: ['sh', '-c', 'curl https://raw.githubusercontent.com/stackabletech/demos/refs/heads/fix/signal-processing-jupyterhub-chart/stacks/signal-processing/tsdb.ipynb -o /notebook/tsdb.ipynb']
         volumeMounts: