fix(helm): Always respect configured registry in image references

Remove the dot-based heuristic that skipped prepending the registry
when the repository contained a dot. This caused explicitly configured
registries (both service-level and global) to be silently ignored for
repositories with dots in their path (e.g. mirror.gcr.io/grafana/loki).

Restores the pre-grafana#19347 behavior of always prepending the registry.
Documents the migration path in the upgrade guide.

Fixes grafana#20663

Author: mag123c

docs/sources/setup/upgrade/_index.md

@@ -45,6 +45,27 @@ Supported clients should check the configuration options for max send message si
 
 ## Helm Chart Upgrades
 
+### Helm Chart 6.50.0 - Always prepend configured registry in image references
+
+The dot-based heuristic introduced in Helm chart 6.50.0 to detect an embedded registry in the `repository` field has been removed. Previously, if the `repository` value contained a dot (e.g., `mirror.gcr.io/grafana/loki`), any configured `registry` (both service-level and `global.imageRegistry`) was silently ignored.
+
+The chart now always prepends the configured registry, which is consistent with the behavior before Helm chart 6.50.0 and with other Grafana Helm charts.
+
+**Action required:** If you are embedding a full registry path in the `repository` field (e.g., `loki.image.repository: private.registry.com/grafana/loki`), split it into separate `registry` and `repository` fields:
+
+```yaml
+# Before (deprecated pattern)
+loki:
+  image:
+    repository: private.registry.com/grafana/loki
+
+# After
+loki:
+  image:
+    registry: private.registry.com
+    repository: grafana/loki
+```
+
 ### Helm Chart 6.50.0 - Respect the global registry in the sidecar image
 
 If you prefixed the sidecar container with a private registry (`sidecar.image.repository`), this is no longer necessary and is deprecated as the global registry is used starting with Helm chart 6.46.1. Therefore please use `global.imageRegistry` or alternatively, `sidecar.image.registry` for more fine-grained control.

production/helm/loki/templates/_helpers.tpl

@@ -166,8 +166,7 @@ Create the name of the service account to use
 
 {{/*
 Base template for building docker image reference
-Determines the final image name, respecting the global registry if defined, unless the local repository
-already contains a full registry (indicated by a dot '.') for backwards-compatibility.
+Always prepends the registry when one is configured (global or service-level).
 It also respects `.digest` as well as `.sha` (deprecated).
 */}}
 {{- define "loki.baseImage" }}
@@ -178,8 +177,7 @@ It also respects `.digest` as well as `.sha` (deprecated).
 {{- $ref := ternary (printf ":%s" (.service.tag | default .defaultVersion | toString)) ($digest) (empty $digest) -}}
 
 {{- $prefix := "" -}}
-{{- $firstSegment := (split "/" $repository)._0 -}}
-{{- if and $registry (not (contains "." $firstSegment)) -}}
+{{- if $registry -}}
 {{- $prefix = printf "%s/" $registry -}}
 {{- end -}}