fix: override image proxy and pull policy for EC airgap installs

- Clear imageProxy.host for EC installs so KOTS rewrites plain image refs
  (online EC) or EC mirrors from local registry (airgap EC)
- Set pullPolicy to IfNotPresent to prevent internet pulls in airgap
- Clear postgresql/redis registry overrides for EC so plain refs are used
- Set docker.io registry in builder so Replicated detects upstream Bitnami
  images and includes them in airgap bundles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: aa-replicated

helmchart.yaml

@@ -13,6 +13,22 @@ spec:
     siteName: repl{{ ConfigOption `site_name`}}
     siteColor: repl{{ ConfigOption `site_color`}}
     customBrandingEnabled: repl{{ LicenseFieldValue `custom_branding_enabled` }}
+    imageProxy:
+      host: ""
+    image:
+      pullPolicy: IfNotPresent
+    postgresql:
+      image:
+        registry: ""
+    redis:
+      image:
+        registry: ""
   builder:
     image:
       tag: latest
+    postgresql:
+      image:
+        registry: "docker.io"
+    redis:
+      image:
+        registry: "docker.io"