fix: standardize global image registry to match other Grafana charts

Fixes #19212

This PR standardizes the global image registry configuration to match
other Grafana charts by supporting the standard global.imageRegistry format
while maintaining backwards compatibility.

Changes:
- Add support for standard global.imageRegistry in _helpers.tpl
- Add global.imageRegistry to values.yaml with proper documentation
- Maintain backwards compatibility with existing global.image.registry
- Update precedence order: global.imageRegistry > global.image.registry > global.registry

Users can now use the standard format across all Grafana charts:
global:
  imageRegistry: my-registry.com

While existing configurations continue to work:
global:
  image:
    registry: my-registry.com

Signed-off-by: puretension <[email protected]>

Author: puretension

production/helm/loki/Chart.yaml

@@ -3,7 +3,7 @@ name: loki
 description: Helm chart for Grafana Loki and Grafana Enterprise Logs supporting monolithic, simple scalable, and microservices modes.
 type: application
 appVersion: 3.5.3
-version: 6.40.0
+version: 6.40.1
 home: https://grafana.github.io/helm-charts
 sources:
   - https://github.com/grafana/loki

production/helm/loki/templates/_helpers.tpl

@@ -160,7 +160,7 @@ Create the name of the service account to use
 Base template for building docker image reference
 */}}
 {{- define "loki.baseImage" }}
-{{- $registry := .global.registry | default .service.registry | default "" -}}
+{{- $registry := .global.imageRegistry | default .global.image.registry | default .global.registry | default .service.registry | default "" -}}
 {{- $repository := .service.repository | default "" -}}
 {{- $ref := ternary (printf ":%s" (.service.tag | default .defaultVersion | toString)) (printf "@%s" .service.digest) (empty .service.digest) -}}
 {{- if and $registry $repository -}}

production/helm/loki/values.yaml

@@ -5,8 +5,10 @@
 kubeVersionOverride: null
 
 global:
+  # -- Overrides the Docker registry globally for all images (standard format)
+  imageRegistry: null
   image:
-    # -- Overrides the Docker registry globally for all images
+    # -- Overrides the Docker registry globally for all images (deprecated, use global.imageRegistry)
     registry: null
   # -- Overrides the priorityClassName for all pods
   priorityClassName: null