feat(chart): make DinD sidecar image configurable via values

Add containerMode.dindImage to allow overriding the hardcoded
docker:dind image in the DinD sidecar container.

This enables users to provide a custom DinD image with pre-configured
daemon.json (e.g., registry mirrors for pull-through caches, insecure
registries) without having to abandon containerMode entirely and
manually define the full pod spec.

Closes #3709

charts/gha-runner-scale-set/templates/_helpers.tpl

@@ -110,7 +110,7 @@ volumeMounts:
 {{- end }}
 
 {{- define "gha-runner-scale-set.dind-container" -}}
-image: docker:dind
+image: {{ .Values.containerMode.dindImage | default "docker:dind" }}
 args:
   - dockerd
   - --host=unix:///var/run/docker.sock

charts/gha-runner-scale-set/values.yaml

@@ -118,6 +118,10 @@ githubConfigSecret:
 ## empty, and configuration should be applied to the template.
 # containerMode:
 #   type: "dind"  ## type can be set to "dind", "kubernetes", or "kubernetes-novolume"
+#   ## Optional: override the default docker:dind image used for the DinD sidecar container.
+#   ## This is useful for using a custom image with pre-configured daemon.json
+#   ## (e.g., registry mirrors, insecure registries) or other customizations.
+#   # dindImage: "docker:dind"
 #   ## the following is required when containerMode.type=kubernetes
 #   kubernetesModeWorkVolumeClaim:
 #     accessModes: ["ReadWriteOnce"]