helm: improvements (#10976)

- fix(ingress): use root context ($) for fullname inside range
- fix(statefulset): use updateStrategy instead of strategy for
mysql/infinity/elasticsearch/opensearch
- feat(mysql): add external mode via mysql.enabled=false with env
MYSQL_HOST/PORT and MYSQL_USER (default root)
- feat(minio/redis): add external mode via *.enabled=false with env
*_HOST/PORT
- feat(global): add global.repo for image registry prefix and
global.imagePullSecrets for all pods
- feat: helper template ragflow.imageRepo to render image with global
repo
- chore(env): allow optional MINIO_HOST, MINIO_PASSWORD, REDIS_PASSWORD
(remove required); keep MYSQL_PASSWORD required
- docs(helm): add helm/README.md and update usage
- refactor(images): apply global repo to all components and init
containers
- test: align test busybox image with global repo helper

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

helm/templates/_helpers.tpl

@@ -42,6 +42,31 @@ app.kubernetes.io/version: {{ .Chart.AppVersion | quote }}
 app.kubernetes.io/managed-by: {{ .Release.Service }}
 {{- end }}
 
+{{/*
+Resolve image repository with optional global repo prefix.
+If .Values.global.repo is set, replace registry part and keep image path.
+Detect existing registry by first segment containing '.' or ':' or being 'localhost'.
+Usage: {{ include "ragflow.imageRepo" (dict "root" . "repo" .Values.foo.image.repository) }}
+*/}}
+{{- define "ragflow.imageRepo" -}}
+{{- $root := .root -}}
+{{- $repo := .repo -}}
+{{- $global := $root.Values.global -}}
+{{- if and $global $global.repo }}
+  {{- $parts := splitList "/" $repo -}}
+  {{- $first := index $parts 0 -}}
+  {{- $hasRegistry := or (regexMatch "\\." $first) (regexMatch ":" $first) (eq $first "localhost") -}}
+  {{- if $hasRegistry -}}
+    {{- $path := join "/" (rest $parts) -}}
+    {{- printf "%s/%s" $global.repo $path -}}
+  {{- else -}}
+    {{- printf "%s/%s" $global.repo $repo -}}
+  {{- end -}}
+{{- else -}}
+  {{- $repo -}}
+{{- end -}}
+{{- end }}
+
 {{/*
 Selector labels
 */}}